Thermophotovoltaic generator

Abstract

A thermophotovoltaic (TPV) generator includes one or more ring modules including a frame formed from a high thermal conductivity material. Each ring module may be attached to another ring module in an axially stacked manner with a gas sealing engagement between them. Each ring module includes, on either a radially inward or radially outward surface thereof, TPV cells having a TPV cell output power no lower than 40% of the product of open circuit voltage and short circuit current at the maximum expected incident photon flux with energies above the TPV cell bandgap,. Each TPV cell converts photons with energy above the TPV cell bandgap thereof from a radiator into electrical energy. Each TPV cell includes a heat mirror on its surface, facing the radiator, which reflects photons with an energy lower than the TPV cell band gap back to the radiator.

Description

THERMOPHOTO VOLTAIC GENERATORCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims benefit of U.S. Provisional Patent Application Serial No. 63 / 710,898, filed October 23, 2024, the disclosure of which is incorporated herein by reference.BACKGROUND

[0002] The following information is provided to assist the reader in understanding technologies disclosed below and the environment in which such technologies may typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the technologies or the background thereof. The disclosure of all references cited herein are incorporated by reference.

[0003] Electric power generation and motive power are currently predominantly obtained from burning fuels. Internal Combustion Engines (ICEs) connected to an alternator or driveshafts form the basis of power systems ranging from grid-level power generators to model aircraft engines. Such systems benefit from the high energy density available from fuels compared to that of electric batteries, enabling long-distance travel and long-duration operation. Such ICEs use either reciprocating pistons on a crankshaft or a turbine.

[0004] These ICEs and their support equipment (such as exhaust recyclers, turbochargers, intercoolers, air filter intakes, fuel delivery, valve drives) require constant maintenance in long-duration operation. For example 4-stroke 80kW aircraft engines require approx. 2000-hour complete rebuilds, and 100-hour oil changes, together with replacement of degraded hoses and fittings. Turbine blades, especially the highest-efficiency varieties, require expensive removal and recoating, and thus complete engine rebuilds, on a regular basis. Constant operation without the attention of dedicated crew appears to be a remote goal. For example, a test mandated by the 2021 US Congress directed the US Navy to achieve 720-hour (1 month) of continuous unattended operation. Two years later this goal was achieved with a 1.5MW diesel. It would appear that unattended operation such as for transoceanic travel is currently infeasible.

[0005] ICE efficiencies are strongly dependent on system load, since the same heavy engine components must be driven independent of load. Thus efficiencies range from less than 10% at light load to a maximum of 55-64% at full load. Such high full-load 55-64% efficiencies are only attained using combined-cycle turbines, which cannot be used in aircraft, or with complex controls on large 2- and 4-stroke marine engines. In general the best full-load ICE efficiencies in diesel generators, diesel trucks, and aircraft turbines is approx. 35-38%.

[0006] A further problem with current ICEs is their highly tailored combustion and exhaust engineering, limiting the range of fuels that can be used. For example, the so-called 'heavy fuels' such as diesel and biofuels being promoted for use across the NATO military sector require extensive re-engineering on ICEs which run on lighter grades of fuel such as gasoline. Gas turbines are an extreme case, with the high-efficiency types heavily dependent upon the type of gas in use, and are intolerant of fuels such as hydrogen which have a higher combustion temperature than current turbine blade coatings can survive.

[0007] Current ICE generator systems have varied responses to sudden load changes; higher loads initially slow generator rotation speed, which then has ripple-on response time effects. Coal-fired or nuclear-powered grid-scale generators rely on massive turbine inertia to accommodate short transients, but then exhibit a long recovery time as boiler power is increased. Diesel or gas turbine generator sets have a poorer immediate response to demand peaks, but then exhibit shorter recovery times to a steady, higher power demand level. An example of a 'rapid-response' high efficiency combined-cycle gas turbine generator is that of the 64% peak efficiency 430MW GE 7HA which has a ramp rate of 150MW / minute Thus this turbine takes approx. 3 minutes to respond to a full-load transient demand.

[0008] Electric generation is also possible from thermal batteries, which store energy for later use. Dense hot bodies, either solid or liquid, may be heated from an electric, fuel, solar, or other energy source. The hot material, upon demand, may be brought into proximity with a thermal-to-electric conversion system, thus providing electric power. Such a 'thermal battery', if sufficiently efficient, would be a useful addition to our energy environment. Such demand response is especially valuable for 'peaking' supply to grid power systems; such peaking power is priced at higher levels than steady 'baseline' power.

[0009] In general, there is a need for a more efficient electric generator able to operate on fuels of high energy density such as biofuel, biogas, NATO heavy fuel, gasoline, natural gas,hydrogen etc. (that is, to be fuel-agnostic). Such a generator should deliver motive power and / or electric power for long periods with a small fuel supply, taking advantage of the high energy density of such fuels. Such a generator should have its high conversion efficiency maintained substantially independent of the applied load. High efficiencies also provide environmental and fuel conservation benefits. Further, such a generator should have high reliability, such that unattended operation for perhaps a year is practical, enabling, for example, long-distance sea propulsion without crews. Moreover, the generator should be capable of rapid response to power demand changes, preferably taking less than 1 minute to deliver into a full-load transient. There is also a need for energy storage so that, for example, wind-generated electric power may be stored for later use and then delivered to the electric grid upon demand.

[0010] Photovoltaic (PV) cells which are designed to collect thermal radiation from a hot radiator instead of sunlight are referenced as “thermophotovoltaic” (TPV) cells. Such TPV energy conversion theoretically promises generation of electricity from radiation at up to 70-80% efficiency for silicon solar cells and even higher efficiency for cells with a higher bandgap energy, and has been attempted many times in the past. While individual system elements have shown promise, no complete and highly efficient generator has been previously reported. The energy losses come from many sources, both inside the TPV cells and outside them. As a consequence complete reported system efficiencies in the range of approximately 2-20% have been reported in connection with a number of TPV cells. See, for example, WE Horne, MD Morgan, VS Sundaram; "IR filters for TPV converter modules", AIP Conf Proc 358, 35-54 (1996) doi.org / 10.1063 / 1.49716, and E.J. Brown, P.F. Baldasaro, S.R. Burger, L.R. Danielson, D.M. DePoy, G.J. Nichols, W.F. Topper, T.D. Rahmlow “The Status of Thermophotovoltaic Energy Conversion Technology at Lockheed Martin Corp.” LM-02K155 January 31, 2003; L.M. Fraas; J.E. Avery; Han Xiang Huan, "Thermophotovoltaics: heat and electric power from low bandgap "solar" cells around gas fired radiant tube burners", Twenty-Ninth IEEE Photovoltaic Specialists Conf., 2002, New Orleans, LA USA DOI: 10, 1109 / PVSC.2002.1190909; and L M Fraas, J E Avery, "TPV Cylindrical Generator for Home Cogeneration using Low NOx Radiant Tube Burner", US Pat. 7196263 (2007); L M Fraas, "Fuel Fired Thermophotovoltaic (TPV) Cylindrical Power Supply and Battery Replacement with Catalytic Matched Emitter or Post IR Emitter Array", US Pat. 85814090 Bl (2013). The competitive potential of TPV has not yet been realized.

[0011] A hot ~500°C radiator emits a range of photon energies, which appear to human eyes a dull red with low overall energies. Such radiation reaches a high-energy greenish-white heat at ~6000°C (the apparent temperature of our Sun's outer shell). The TPV cell type used would ideally be matched to radiator temperature, such that the peak radiated photon energies are converted by the cell to electron flow. However thermal radiated photon energies span a wide range, thus radiation-to-electric conversion in a simple PV cell with a single 'bandgap' energy is inefficient. Such a PV cell cannot use photon energies lower than its bandgap energy, and wastes the fraction of photon energy lying above its bandgap energy.

[0012] This wastage of both low and high radiant energies is apparent in solar PV systems, which must accept multiple photon energies in solar radiation ranging from infrared to ultraviolet. While efficiencies of solar PV systems can reach almost 30% with single-junction cells and 45% with multiple-junction cells, the wastage of radiant energies higher than cell bandgaps, combined with the loss of all radiation with lower energies than the bandgaps, means that the theoretical maximum of -80% PV solar cell conversion efficiency is not attainable. High efficiency is however possible using high intensity of a single radiant energy. See, for example, Nils-Peter Harder and Peter Wurfel, Theoretical limits of thermophotovoltaic solar energy conversion; Semiconductor Sci. Technol. 18 (2003) S151-S157 stacks.iop.org / SST / 18 / S151 where -80% efficiency is calculated. A TPV system is capable of approaching ideal high efficiency values since it operates its PV cells over a narrow band of radiant energies, and especially so if the intensity of this radiation is high, yielding higher output voltages than do low intensities in a well-designed cell. The TPV system should select the best radiant energy band for its TPV cell while efficiently recycling unwanted bands back to the hot radiator.

[0013] A crucial feature of TPV conversion is thus efficient recycling of unwanted and inefficient radiation back to the radiator, for later emission in a more useful energy band. If the radiator cannot receive or re-absorb this recycled energy, it is lost. Thus gaseous high-transparency emitters, or solid “selective emitters” which are less-emitting (and less-absorbing) at unwanted energies, are inferior to solid, highly-absorbing and energyindependent emitters, unless their emission selectivity is extremely high. No such highly selective emitter is currently known which can operate long-term at the required TPV temperatures of over 1000°C.

[0014] TPV cells in an efficient system are exposed to a high energy flux. Using the Stefan-Boltzmann law, the energy flux radiated from a ~1900°C (-2200K) radiator, ignoring ambient return flux and assuming unity radiator emittance, is Q;4 -2Q = GT W-m Equation 1-8wherein G = 5.67 * 10 , and T = temperature (K). In the below discussion, f = the fraction of 4 radiated photon energies above the TPV cell bandgap. Thus, Q = 5.67 * 23 * 10 =-21.33MW-m . For a TPV cell of 1cm x 1cm, the incident radiant flux is thus approximately 133W. This is approximately 1000 times the power incident on standard PV cells exposed to sunlight. If the usable fraction of radiated photons f = 0.1, that small TPV cell's output could be of the order 10W. TPV cells must be designed specifically to operate efficiently at such high power levels.

[0015] TPV radiation recycling requirements are illustrated by the fact that in a typical TPV system only perhaps 10% of the incident radiation on a cell structure may be usefully above the cell's bandgap energy. The radiation efficiency formula, assuming the radiator completely absorbs the TPV cell's recycled radiation and that the TPV cell completely absorbs the higher-energy ‘Useful’ photons, is[Useful radiation into PV cell] / [Radiator heat net emission] = r|r-e= 1 / [ R + (l-R) / f ] Equation 2wherein R = the recycling reflectance at photon energies below the TPV cell bandgap energy.

[0016] Thus for R = 0.99 (that is, 1% reflection loss) and f = 0.1 (that is, 10% of radiator radiation is potentially useful), r|r.e~ 0.92. Thus 0.1 fraction of useful photons and 1% recycling reflection loss yield ~8% effective system loss since on average 9 radiation round trips are required before the radiation gets recycled into a usable energy range.

[0017] Recent reports indicate TPV efficiencies of 30% - 40%. T. C. Narayan et al., "World record demonstration of > 30% thermophotovoltaic conversion efficiency," 2020 47th IEEE Photovoltaic Specialists Conference (PVSC), Calgary, AB, Canada, 2020, pp. 1792-1795 doi.org / 10.1109 / PVSC45281.2020.9300768 and LaPotin, A., Schulte, K.L., Steiner, M.A. et al. "Thermophotovoltaic efficiency of 40%" Nature 604, 287-291 (2022)doi.org / 10.1038 / s41586-022-04473-y. Those special testbed results are encouraging. However the graphs in those studies show cell reflectance of approximately 40% in the photon energy band that they need to accept, and a low-energy photon reflectance of 90-95%. Taking an average value of R - 0.9 (that is, a reflection loss of -10%) and assuming f = 0.1 again, r|r.e- 0.53, for an approx. 50% system loss before consideration of PV cell efficiencies. Even at the low radiation intensities in those studies, overall TPV system efficiency is thus significantly reduced by such an optical loss. In addition, the above formula assumes that incident ‘Useful radiation’ is completely absorbed. The reported -40% TPV cell reflectance in the desirable photon energy band further halves available system efficiency to the 10-20% levels attained in earlier complete system studies as described above. Moreover, the above reported cell efficiencies degrade above 2.5W electrical output for the studied ~lcm x 1cm cells due to cell exterior and interior losses, which is a factor of 4 below the required power density for an efficient TPV system.

[0018] TPV generation has been proposed with a central radiator surrounded by TPV cells of 400 times the radiator area. See US Patent No. 5,611,870. In such a system, however, unwanted photons reflected from TPV cells have a very low probability of efficiently being returned to the radiator, yielding low overall system efficiency. WE Home, MD Morgan, VS Sundaram; "IR filters for TPV converter modules", AIP Conf Proc 358, 35-54 (1996) doi.org / 10.1063 / 1.49716.

[0019] PV cell output is typically less than 1 Volt. To reach DC bus levels of, for example, 250 or 600V, cells may be placed in series connection. In that regard, 250V may be reached with, for example, 360 cells in series, wherein each cell generates 0.7-0.8V in the case of silicon PV cells. However cells vary from lot to lot, and individually. In addition radiator intensity will vary across the radiator area. The cells with lowest output current dominate series string output, again degrading efficiency. Series cell connections for TPV generation have included large arrays of cells glued onto a cylinder, small series-connected monolithic structures, and individually series-connected cells on a larger flat substrate. See examples in Bitnar, B., "Silicon, germanium and silicon / germanium photocells for thermophotovoltaics applications" Semiconductor Sci. Technol., 18 (2003) S221-S227 stacks.iop.org / SST / 18 / S221, Wilt D, Wehrer R, Palmisiano M, Wanlass M, Murray C "Monolithic Interconnected Modules (MIMs) for Thermophotovoltaic Energy Conversion" Semiconductor Sci. Technol., April (2003) DOI: 10.1088 / 0268-1242 / 18 / 5 / 310 ], and E.J.Brown, P.F. Baldasaro, S.R. Burger, L.R. Danielson, D.M. DePoy, G.J. Nichols, W.F. Topper, T.D. Rahmlow "The Status of Thermophotovoltaic Energy Conversion Technology at Lockheed Martin Corp." LM-02K155 January 31, 2003.

[0020] Development of a high-efficiency TPV system competitive with ICE's has thus been an unattained goal.SUMMARY

[0021] A thermophotovoltaic (TPV) generator includes one or more ring modules including a frame formed from a high thermal conductivity material formed as a ring. Each of the one or more ring modules is adapted on each axial end thereof to be attached to another of the one or more ring modules in an axially stacked manner with a gas sealing engagement between them. Each of the one or more ring modules includes a plurality of TPV cells, which have a TPV cell output power no lower than 40% of the product of open circuit voltage and short circuit current at the maximum expected incident photon flux above the TPV cell bandgap, and are attached on either a radially inner surface of the ring or on a radially outer surface of the ring. Each of the TPV cells is configured to convert photons with energy above the TPV cell bandgap thereof from a radiator into electrical energy. Each of the TPV cells includes a heat mirror on a surface thereof, facing the radiator, which reflects photons with an energy lower than the TPV cell band gap back to the radiator. The heat mirror may also reflect photons that would yield above-bandgap high-energy losses. The radiator is positionable within a central volume of the one or more ring modules when the plurality of TPV cells are attached on the radially inner surface of the ring. The radiator is positioned radially outside of a perimeter of the one or more ring modules when the plurality of TPV cells are attached on the radially outer surface of the ring such that the TPV cells are oriented to face a radiating surface of the radiator. The TPV cells are mounted on polygonal surfaces of the ring facing the radiator. The number of ring modules attached in the axially stacked manner is determined based upon a predetermined maximum power level for the stacked ring modules forming a main module.

[0022] In a number of embodiments, the plurality of TPV cells face toward the radiating surface of the radiator with a geometry such that photons reflected from a surface of each of the plurality of TPV cells have a probability of failure to directly return photons to the radiator no greater than double the TPV cell low-energy photon reflectance loss. In a numberof embodiments, gaps between TPV cells include a reflective coating such that the overall ring module and main module reflectance loss is no greater than double the TPV cell reflectance loss for photon energies lower than the TPV cell bandgap.

[0023] Each of the one or more ring modules may, for example, include a DC bus segment, a control system, a communication system, and circuitry in connection with the DC bus segment and the communication system. The circuitry is configured to boost the outputs of the plurality of TPV cells of the ring modules to a higher de bus level to be output from the TPV generator. The DC bus segment of each of the one or more ring modules is placed in electrical connection with the DC bus segment of each of another of the one or more ring modules when attached in the stacked manner to form the main module.

[0024] In a number of embodiments, the communication system includes a control line segment. The control line segment of each of the one or more ring modules is placed in electrical connection with the control line segment of each of another of the one or more ring modules when attached in the stacked manner. The communication system of each of the one or more ring modules may be configured to place the ring module in communication with a central electronic circuitry comprising a central control system in a wired or wireless manner.

[0025] In a number of embodiments, the TPV generator includes a plurality of the ring modules which are attached in the stacked manner to form a main module and DC bus power output from the TPV generator is fed by one or more of the plurality of ring modules to the main module DC bus output using one or more DC bus levels to attain a final high de bus voltage. The TPV generator may include a plurality of main modules and wherein each of the plurality of main modules is controlled via signals communicated between the communication systems of each of the plurality of ring modules of each of the plurality of main modules and the central electronic circuitry.

[0026] The TPV cells of each of the one or more ring modules may, for example, be attached on the radially outer surface of the ring module in the case of an inner radiator. Each of the one or more ring modules may further include circuitry configured to boost the outputs of the plurality of TPV cells and for control. The circuitry may be positioned on an inside of the ring in the case of an outer radiator. In such embodiments with an outer radiator, the generator may, for example, be used to generate electric power from heat radiated from the radiator to operate as a thermal battery.

[0027] Each of the one or more ring modules may include an airflow cooling system including radiator fins with airflow providing heat transfer. Each of the one or more ring modules may include a coolant fluid conduit. The TPV generator may further includes a coolant return conduit via which a coolant fluid which has passed through coolant fluid conduits in each of the one or more ring modules is returned to an external heat exchanger. In a number of embodiments, the TPV generator further includes a coolant return manifold in connection with a distal end ring module of the one or more ring modules. The coolant return manifold includes a conduit to receive the coolant fluid from the coolant fluid conduit of the distal end ring module. The coolant return manifold is configured to return the coolant fluid to the external heat exchanger via the coolant return conduit.

[0028] In a number of embodiments, the TPV cells of each of the one or more ring modules are attached on the radially inner surface of the ring facing an inner radiator. Each of the one or more ring modules may further include circuitry configured to boost the outputs of the plurality of TPV cells and for control. The circuitry is positioned on an exterior of the ring, the TPV generator being used to generate electric power from heat radiated from the radiator. As described above, each of the one or more ring modules may include a coolant fluid conduit. The TPV generator may further includes a coolant return conduit via which a coolant fluid which has passed through coolant fluid conduits in each of the one or more ring modules is returned to an external heat exchanger. In a number of embodiments, the TPV generator further includes a coolant return manifold in connection with a distal end ring module of the one or more ring modules. The coolant return manifold includes a conduit to receive the coolant fluid from the coolant fluid conduit of the distal end ring module. The coolant return manifold is configured to return the coolant fluid to the external heat exchanger via the coolant return conduit.

[0029] In a number of embodiments, the radiator is surrounded by a fill gas. The fill gas is at a controlled pressure. The fill gas may, for example, be selected to assist in radiator evaporation suppression and in suppression of radiator degradation. The fill gas may be inert, or may include a reactant to limit deposition of radiator products on the heat mirror surface.

[0030] A sealing engagement may be created between each of the one or more ring modules when axially stacked which provides a seal to maintain a controlled atmosphere between the radiator and the plurality of TPV cells of each of the one or more ring modules.

[0031] In a number of embodiments, the radiator is heated by fuel combustion. Oxidizer gas which is supplied to the radiator may be preheated by heat exchange with exhaust gas from combustion of the fuel. In a number of embodiments, the oxidizer gas is supplied to a combustion chamber of the radiator and is pressurized with a fan powered either from the DC bus power of the TPV generator or from the exhaust gas using a turbocharger or from 'ram air' obtained from high-speed travel through air.

[0032] The TPV generator may further include a controlled-atmosphere injection ring attachable to one of the one or more ring modules via which a controlled-atmosphere can be introduced to a central volume.

[0033] The TPV generator may further include a fuel supply ring via which fuel is supplied to the radiator.

[0034] In a number of embodiments, the main module is attachable to a main manifold which can interconnected with a plurality of main modules mounted on such main manifolds. The main module may also, for example, be attachable to a main manifold which can accept a plurality of main modules.

[0035] In a number of embodiments, the high thermal conductivity material has a thermal conductivity of no less than 100W / (m-K). The heat mirror may, for example, include a combination of one or more of a metallic nanostructured heat mirror, reflection-enhancing thin film, transmission-enhancing thin film, and a broad-band multilayer dielectric stack heat mirror.

[0036] In another aspect, a TPV generator includes a plurality of main modules hereof attached to a main manifold. Each of the main modules may, for example, include a fuel supply ring via which fuel is supplied to the radiator thereof and wherein the fuel supply ring is attached to the main manifold. The TPV generator may further include a fuel supply system in fluid connection with the main manifold. In a number of embodiments, the TPV generator further includes at least a main manifold, which independently has a plurality of main modules mounted on main manifolds attached thereto.

[0037] In a further aspect, a method of generating electricity includes providing a TPV generator as set forth herein.

[0038] The present devices, systems, and methods, along with the attributes and attendant advantages thereof, will best be appreciated and understood in view of the following detailed description taken in conjunction with the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIG. 1A illustrates a side view of an embodiment of a fuel-driven TPV generator hereof including a single TPV cell generator module hereof.

[0040] FIG. IB illustrates a section A-A, side, cross-sectional view of the generator of FIG. 1A.

[0041] FIG. 2A illustrates an isometric view of the generator of FIG. 1 A.

[0042] FIG. 2B illustrates an isometric view of the generator of FIG. 1 A.

[0043] FIG. 3 illustrates a bottom view of the generator of FIG. 1 A.

[0044] FIG. 4A illustrates an isometric view of the generator of FIG. 1 A with heat sinking fins attached to the TPV cell ring modules thereof.

[0045] FIG. 4B illustrates a side view of the generator of FIG. 4A.

[0046] FIG. 4C illustrates another isometric view of the generator of FIG. 4A.

[0047] FIG. 4D illustrates an isometric, exploded view of the generator of FIG. 4A.

[0048] FIG. 4E illustrates another isometric, exploded view of the generator of FIG. 4A.

[0049] FIG. 4F illustrates a side, exploded view of the generator of FIG. 4 A.

[0050] FIG. 4G illustrates a side, exploded view of the generator of FIG. 4A.

[0051] FIG. 4H illustrates an embodiment of a TPV generator hereof including a plurality of the TPV cell generator ring modules as illustrated in FIG. 4A which are interconnected via stacking to provide a desired total power output.

[0052] FIG. 41 illustrates another isometric view of the stacked-module, TPV generator of FIG. 4H.

[0053] FIG4J illustrates several side views and a bottom end view of the stacked-module, TPV generator of FIG. 4H.

[0054] FIG. 5 A illustrates an isometric view of an assembled system including a TPV generator of FIG. 1 A attached to an embodiment of a modular manifold section hereof.

[0055] FIG. 5B illustrates another isometric view of the system of FIG. 5A.

[0056] FIG. 5C illustrates another isometric view of the system of FIG. 5 A.

[0057] FIG. 5D illustrates a side view of the system of FIG. 5 A.

[0058] FIG. 5E illustrates an exploded isometric view of the system of FIG. 5A

[0059] FIG. 5F illustrates another exploded isometric view of the system of FIG. 5A.

[0060] FIG. 5G illustrates a side exploded view of the system of FIG. 5 A.

[0061] FIG. 6 A illustrates an isometric view of a first segment of the manifold section of the system FIG. 5A, which is connectible to a TPV generator as illustrates in FIG. 1A or FIG. 4A.

[0062] FIG. 6B illustrates another isometric view of the first segment of FIG. 6A.

[0063] FIG. 6C illustrates a top view of the first segment of FIG. 6 A.

[0064] FIG. 6D illustrates a bottom view of the first segment of FIG. 6 A.

[0065] FIG. 6E illustrates a side view of the first segment of FIG. 6A.

[0066] FIG. 6F illustrates another side view of the first segment of FIG. 6 A.

[0067] FIG. 6G illustrates another side view of the first segment of FIG. 6 A.

[0068] FIG. 6H illustrates an isometric view of two of the first segments of FIG. 6A in alignment for interconnection.

[0069] FIG. 7 illustrates a representative embodiment of a boost circuit for use herein.

[0070] FIG. 8 illustrates blackbody spectral energy fractions for a Si TPV cell as a function of temperature.

[0071] FIG. 9 illustrates TPV radiator evaporation rates as represented by film thickness (nm / year) as a function of radiator temperature for carbon (C) and tungsten (W).

[0072] FIG. 10 illustrates inert gas thermal conductivity as a function of temperature for Helium (He), Argon (Ar), Krypton (Kr), and Xenon (Xe).

[0073] FIG. 11 illustrates another embodiment of a TPV module hereof with cells on the outside of a heatsink for use in 'thermal battery' configurations.

[0074] FIG. 12 illustrates schematically an embodiment of a reflection recycling geometry hereof for a TPV module with TPV cells surrounding a central radiator.

[0075] FIG. 13 illustrates the probability of one-bounce reflectance as a function of geometric g / d ratio for a simple radiator configuration with opposing flat surfaces.

[0076] FIG. 14 illustrates geometric recycling loss and heat mirror long-wave reflectance as a function of geometric g / d ratio for the case of opposing flat surfaces.

[0077] FIG. 15A illustrates an isometric view of an embodiment of a radiator geometry permitting relatively large gaps 'g' relative to radiator size 'd', but with small optical losses.

[0078] FIG. 15B illustrates a cross-sectional view of the radiator geometry of FIG. 15 A.DETAILED DESCRIPTION

[0079] It will be readily understood that the components of the embodiments, as generally described and illustrated in the figures herein, may be arranged and designed in a wide variety of different configurations in addition to the described representative embodiments. Thus, the following more detailed description of the representative embodiments, as illustrated in the figures, is not intended to limit the scope of the embodiments, as claimed, but is merely illustrative of representative embodiments.

[0080] Reference throughout this specification to “one embodiment” or “an embodiment” (or the like) means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” or the like in various places throughout this specification are not necessarily all referring to the same embodiment.

[0081] Furthermore, described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided to give a thorough understanding of embodiments. One skilled in the relevant art will recognize, however, that the various embodiments can be practiced without one or more of the specific details, or with other methods, components, materials, et cetera. In other instances, well known structures, materials, or operations are not shown or described in detail to avoid obfuscation.

[0082] As used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. Thus, for example, reference to “a cell” includes a plurality of such cells and equivalents thereof known to those skilled in the art, and so forth, and reference to “the cell” is a reference to one or more such cells and equivalents thereof known to those skilled in the art, and so forth. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each separate value, as well as intermediate ranges, are incorporated into the specification as if individually recited herein. Unless clearly contraindicated by the text, use of terms such as “approximately,” “about,” and the like in connection with a value indicates to values with 10%, and more typically within 5% of the value. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contraindicated by the text.

[0083] The terms “electronic circuitry,” “circuitry” or “circuit,” as used herein include, but are not limited to, hardware, firmware, software, or combinations of each to perform a function(s) or an action(s). For example, based on a desired feature or need, a circuit may include a software controlled microprocessor, discrete logic such as an application specific integrated circuit (ASIC), or other programmed logic device. Circuits may include field programmable gate arrays (FPGAs) which are integrated circuits often sold off-the-shelf. Such devices are referred to as 'field programmable' because the device provide one the ability to reconfigure the hardware to meet specific use case requirements after the manufacturing process. A circuit may also be fully embodied as software. As used herein, “circuit” is considered synonymous with “logic.” The term “logic,” as used herein includes, but is not limited to, hardware, firmware, software, or combinations of each to perform a function(s) or an action(s), or to cause a function or action from another component. Forexample, based on a desired application or need, logic may include a software controlled microprocessor, discrete logic such as an ASIC, or other programmed logic device. Logic may also be fully embodied as software.

[0084] The term “processor," as used herein includes, but is not limited to, one or more of virtually any number of processor systems or stand-alone processors, such as microprocessors, microcontrollers, central processing units (CPUs), and digital signal processors (DSPs), in any combination. The processor may be associated with various other circuits that support operation of the processor, such as random access memory (RAM), readonly memory (ROM), programmable read-only memory (PROM), erasable programmable read only memory (EPROM), clocks, decoders, memory controllers, or interrupt controllers, etc. These support circuits may be internal or external to the processor or its associated electronic packaging. The support circuits are in operative communication with the processor. The support circuits are not necessarily shown separate from the processor in block diagrams or other drawings.

[0085] The term “memory system” refers to a collection of electronic components that store data and instructions. In computerized systems, a processor system can quickly access information stored in a memory system. Memory allows storage and retrieval of information and may, for example, include primary memory and secondary memory. Primary memory includes, for example, RAM, cache memory, etc. Secondary memory includes, for example, hard drives, hard disk drives etc.

[0086] The term “controller,” as used herein includes, but is not limited to, any circuit or device that coordinates and controls the operation of one or more input and / or output devices. A controller may, for example, include a device having one or more processors, microprocessors, or central processing units capable of being programmed to perform functions.

[0087] The term “software,” as used herein includes, but is not limited to, one or more computer readable or executable instructions that cause a computer or other electronic device to perform functions, actions, or behave in a desired manner. The instructions may be embodied in various forms such as routines, algorithms, modules, or programs including separate applications or code from dynamically linked libraries. Software may also be implemented in various forms such as a stand-alone program, a function call, a servlet, anapplet, instructions stored in a memory, part of an operating system or other types of executable instructions. It will be appreciated by one of ordinary skill in the art that the form of software is dependent on, for example, requirements of a desired application, the environment it runs on, or the desires of a designer / programmer or the like.

[0088] As used herein, the term “approximately” when used in connection with a value means within 5%, within 2%, or within 1% of the value unless otherwise indicated herein or otherwise clearly contraindicated by the text. As used herein the term “and / or” means one of or both of an entity. Thus, A and / or B means A or B, or both A and B.

[0089] It is not sufficient to measure TPV cell performance in an isolated testbed at limited input powers. It is necessary to address TPV full-system losses to achieve significant increases in efficiency. A number of embodiments, integrated devices, systems and methods hereof reduce multiple TPV loss mechanisms, thereby delivering previously unattainable levels of efficiency.

[0090] In that regard, in a number of embodiments specialized TPV cells capable of high output power and high efficiency are used herein. For example, TPV cell surfaces with “heat mirrors” of a combination of one or more of nanostructured metal, reflectance- and transmittance-enhancing thin films, and multilayer wavelength-selective thin film filters may be used to accomplish high-efficiency photon recycling combined with selectivity for high transmission of the “useful” high-energy photons.

[0091] In the case of a thermal battery where a cylindrical array of TPV cells face outwards into a large grouping of hot radiating bodies, the reflected photons returned from TPV cells such as unusable low-energy photons, plus photons that would yield above-bandgap high-energy losses, will certainly arrive back at those radiating bodies without further reflections from neighboring TPV cells. However in the converse case of TPV cells facing inwards towards a central radiator, optical ‘view factors’ play an important role in TPV system design. TPV photon recycling demands efficient two-way photon exchange between TPV cells and the hot radiator without incurring unwanted numbers of lossy reflections, and devices, systems, and methods hereof provide an appropriate view factor to significantly increase efficiency.

[0092] A TPV cell array must be mounted on a heatsink; for example if a module generates 300W of electrical power, inherent cell losses combined with front-surface reflection losseswill yield perhaps 200W cell heating. PV cell efficiencies fall as temperature rises, so good heatsinking of the cell array is important. Such heatsinking requires a base material of high thermal conductivity to the means of dissipating heat such as cooling fins and coolant channels. Suitable materials include, for example, metals such as silver, copper, aluminum and their alloys; ceramics such as aluminum nitride and beryllium oxide, and composite materials such as aluminum-infused silicon carbide. Typically, weight / performance / cost tradeoffs will dominate the heatsink material choice. In general since a rise of silicon TPV cell temperature results in a loss of output power at the rate of ~4% per 10°C, cell heating to 20°C above a reference temperature results in an approximate 10% loss of available cell output. Thus a 3kW system with 55% initial efficiency at its rated ambient temperature would be degraded to an efficiency of 50% and a 2.7kW output power when operating at a temperature 20°C higher. If possible, cooling TPV cells below ambient temperature would further enhance the efficiencies promised by this TPV technology, so aircraft at cryogenic cruising-altitude height would attain correspondingly higher efficiencies.

[0093] TPV conversion efficiencies can be degraded by TPV ring module construction defects, such as the gaps between neighboring cells. TPV cell geometry is typically that of a thin rectangle. Thus a cell array incorporates joints in between cells, yielding possible reflectance loss due to radiation absorption in the narrow joints. As an example, a basic 40mm x 20mm PV cell with 0.1mm gap to its neighboring cells results in approximately 1% reflection loss. This loss must be added to the overall low-energy reflectance loss R in the above formula, thus becomes a significant limit to the attainment of high overall efficiency. For the same gap size, smaller cells such as those in the above-quoted 30% and 40% efficiency papers of approx. 10mm x 10mm area may exhibit an even higher 2% loss. Devices, systems, and methods of alleviating such losses are described here.

[0094] As described above, multiple TPV cells may be connected in series to achieve a desired DC bus level. However cells vary from lot to lot, and individually. In addition radiator intensity will vary across the radiator area. The cells with lowest output current dominate series string output, again degrading efficiency. In a number of embodiments or devices, systems, and methods hereof is described individual cells, or small collections of similar cells, being able to contribute to their highest individual capabilities into TPV system output power.

[0095] At high temperatures, radiator material evaporation becomes a critical factor; clouding PV cell surfaces, degrading efficiency over time. In addition, the space between TPV cells and radiator should exhibit low thermal conductivity and convection to minimize non-radiative heat losses. In a number of embodiments, devices, systems, and methods hereof employ design principles to attain long-duration and high system efficiency, with suppression of evaporation combined with low thermal conductivity between radiator and TPV cells.

[0096] With fuel being burnt to achieve high temperature, it would be beneficial to have associated oxidizing gas (in most cases, air) preheated (for example, from exhaust gases) to achieve an optimal total heating efficiency. In a fuel-burning TPV system this may, for example, be achieved via recycling exhaust heat in some fashion. High oxidizer gas flow is also desirable in the TPV case to attain high energy generation rate and high uniformity along the radiator structure. An elevated-pressure combustion chamber design with hot oxidizer gas / air injection into the combustion chamber, and with residual exhaust energy able to be used to turbocharge incoming air; or preheat in a countercurrent heat exchanger, using a parasitic fan on the oxidizer input similarly to supercharger operation may, for example, be used to attain a high and uniform energy generation rate.

[0097] TPV generator devices, systems, and methods hereof exhibit highest efficiency at their maximum power output and maximum radiator temperature. However radiator and other component lifetime is reduced at such temperatures, so lower-temperature operation is desirable to attain long times before required preventive maintenance. On the other hand, if a TPV system is scaled at its maximum output power to deliver acceptable efficiency for long maintenance periods, then system efficiency will be low in periods of low power demand. Such low-load efficiency degradation is also present in ICEs of all designs, which must continue moving massive components independent of load. In a number of embodiments of devices, systems and methods hereof, main modular elements of a complete system can be isolated such that output capability is tailored to load requirements, with high efficiency maintained throughout the anticipated system load range. Load spikes may be accommodated with a transient radiator temperature boost on active main modules, followed by further main modular elements being brought online. Such controls may operate valving on the fuel lines and input oxidizer fan controls for individual main modules of a complete generator system, such that dormant main modules are turned off, or maintained at an intermediate temperature with a small fuel supply, thus enhancing response speed to load demands.

[0098] Thus modular construction permits system control to maintain high efficiency over a wide load range. In addition, the modular principle can be pushed down from the main module level into the individual TPV cell array level, wherein ring modules of TPV cells, mounted on their heatsinks, may be stacked to attain a desired output power level in each main module. A control system to operate such a complete generator system would thus monitor load requirements and stage the activation of the various main modules to match the load vs. time profile to yield the optimum system efficiency while limiting system degradation due to radiator evaporation, and monitoring of main module status to enable predictive maintenance.

[0099] The control system may also activate a gas purge or gas pump and refill operation at such periods as to maintain sufficient gas purity in between radiator and TPV cells. For example, excessive oxygen content may accelerate radiator degradation, so a periodic renewal of gas fill purity may be desirable. In a number of embodiments, oxygen sensing devices may, for example, be connected to the gas fill line, similar to those used in automobile exhaust systems, and operated either continuously or on demand from the control system.

[0100] In a number of embodiments, a control system hereof may also monitor the ring modules. Each attached ring module electronic circuit may, for example, incorporate performance data such as temperature, output voltage, and output current, and transmit that data to the central system controller. Such information may be used, for example, to determine that a ring module is experiencing excessive heat rise compared to its neighbors, which may be the result of a TPV cell fault, triggering shutdown of that main module and notification of a need for replacement of the defective ring module. The required communications link may be of any type including commonly used wire standards such as USB, I2C, RS-485, RS-232, Ethernet, Profinet, CAN; or IR LED or fiber-optic links; or wireless communications such as ZigBee, Wi-Fi, WirelessHART, and Bluetooth; or ultrasonics. Communications may also be achieved using signal modulation over the bus power line. Separately wired communication is discussed in a number of embodiments hereof, but one skilled in the art will appreciate that other communication modes are possible.

[0101] In a number of embodiments, TPV generators hereof may be formed from interconnectible main modules including TPV cells mounted on ring modules. TPV cells hereof may, for example, be made economically and assembled into such modular arrayswhile retaining high system efficiency. Such cells generate power which, boosted efficiently to a higher-voltage de "bus" level can, for example, be used directly or converted with high-efficiency semiconductor inverters to ac for traction motors or ac line distribution.

[0102] TPV cells for use herein may, for example, be designed and selected to exhibit low losses and high output cell voltages (for example, at electrical output power densities of between 5 and 20 W per sq cm). TPV cell series conduction losses may, for example, be less than 5% of peak output power at the rated radiator temperature and ring module geometry. That possible 5% cell conduction loss directly degrades total system output power and efficiency in a similar manner to the previously discussed effect of cell temperature, and becomes one of the numerous accumulating TPV system efficiency degradation mechanisms. In a practical system series conduction losses should be held to less than or equal to 40% of peak output power, yielding -33% system efficiency similar to the peak efficiency ratings of most internal combustion engines; but preferably would be of the order of 0.1% when system performance optimization is a priority. This series conduction loss derives from internal and contact resistances to the TPV cell's semiconductor doped regions and from resistance of the metal traces carrying current from the cell; both are desirably minimized.

[0103] Generators hereof provide a number of improvements enabling a more efficient electric generator than currently available, and which are able to operate on fuels of high energy density such as biofuel, biogas, NATO heavy fuel, gasoline, natural gas, hydrogen etc. (that is, to be fuel-agnostic). The generators hereof may deliver motive power and / or electric power for long periods with a small fuel supply, taking advantage of the high energy density of such fuels. The high conversion efficiency of generators hereof may be maintained substantially independent of the applied load. The generators hereof may be highly reliable, providing for long unattended periods of operation and enabling, for example, long-distance sea propulsion without crews. Moreover, the generators hereof may provide rapid response to power demand changes. Further benefits of the generators hereof include silent operation, small volume, and low weight. Such features may be helpful in, for example, aircraft, train, ship, bus, truck, and automobile power systems. As an example, portable gasoline generators able to deliver 2kW currently weigh approximately 22kg without their fuel. A silent generator hereof, with a similar power rating, will weigh approximately 5kg and consume approximately half the fuel.

[0104] In a number of embodiments, generators hereof include a long-lived radiator facing TPV cells in an environment yielding low radiation and heat losses, and minimal longterm PV cell obscuration. The TPV cells provide high efficiency at high illumination levels and high recycling efficiency for unwanted radiation components. Further, TPV cell output power may be optimized such that each cell or cell module contributes its maximum possible power output to a higher-voltage de bus. In that regard, in a number of embodiments of systems hereof, individual TPV cells, or small collections of similar cells, are able to contribute to their highest individual capabilities into TPV system output power.

[0105] In a number of embodiments, generators hereof may provide a system for recuperating combustion exhaust heat, using at least one of provision of heat to the incoming combustion oxidizing gas, and of powering a turbocharger or parasitic electrically-driven supercharging fan for compression of that oxidizer gas before it is heated.

[0106] To assist in meeting current needs for energy storage, generators hereof are also suitable for use in a ‘thermal battery’ such that stored heat in hot materials can be converted with the generator modules hereof to electricity upon demand, and with efficiency rivalling the charge / discharge cycle efficiency attainable from current electrochemical batteries.

[0107] Generator systems hereof, with alleviation of their potential loss mechanisms, may, for example, be capable of efficiencies greater than 55% total efficiency when burning fuels, and similar or higher efficiency when used as a thermal battery generator. This is competitive with many of the best thermal-to-electric generator systems to date. Further optimization could reach closer to the theoretical 70-80% levels of efficiency.

[0108] With a limited choice of highly selective emitters which can operate long-term at the required TPV temperatures of over 1000°C being available, primarily non-selective radiators combined with selective TPV cell constructions are used herein. TPV generator systems hereof include a number of improvements compared to previously studied systems, including, but not limited to, controlled and high-efficiency reflection of recyclable photons back to the radiator, connection of TPV cells onto a modular framework, use of TPV cells with high efficiency at high power levels, controlled radiator evaporative contamination, and limitation of series string losses using voltage boost electronics hereof.

[0109] FIGS. 1A through 4G illustrate several views of a representative embodiment of a fuel-driven TPV generator [5] including a single TPV cell generator module or ring module [9] hereof (sometimes referred to simply as “modules [9]” herein). As described above, embodiments of electric generators hereof may include multiple TPV cell generator ring modules [9] (see, FIGS. 4H through 4 J), wherein each TPV cell generator ring module [9] includes multiple individual TPV cells

[0012] mounted on a frame

[0013] formed from a heatsinking material which may include heatsinking fins

[0013] as illustrated in FIGS. 4 A through 5G. and with associated heat exchanging or heat dissipation structures [11, 13, 26, 27], which are discussed further below. In a number of embodiments, the heatsinking materials of frame

[0013] is a high thermal conductivity material the material, such that temperature drop between a surface of TPV cells

[0012] and heat dissipation structures such as structures [11, 13, 26, 17] is less than 60 °C under full-power conditions. Such high thermal conductivity materials may, for example, exhibit a thermal conductivity no lower than 100W / (m-K). In a number of embodiments, the material of frame

[0013] is a metal. Ring modules [9] may, for example, be interconnected by stacking as shown in FIGS. 4H through 4J to yield a desired total electrical power output. A plurality of main modules or main modules [5a] may be connected to a main manifold 100 (as illustrated in FIG. 4H) via main manifold mount surfaces

[0024] thereof as shown in FIGS. 1A and IB. As discussed further below, such a main manifold [100a] may, for example, be formed from a plurality of manifold segments [110a, 140a, 160a] as illustrated in FIGS. 5 A through 6H.

[0110] Referring, for example, to FIG. IB, in more detail, TPV cells

[0012] of each module [9] are disposed in a ring (typically in the form of a polygon such as an octagon) surrounding a radiator

[0014] , Since TPV cells are made from oriented crystal materials which require cooling for best efficiency, their surfaces are often flat and must then be mounted on a flat surface. Thus the ring module [9] should permit cell mounting onto flat surfaces of frames or supports that yield good heatsinking capability, with holes in the flat polygonal surfaces permitting power output wiring from such cells to be brought to the outer ring surface where it can connect to power output circuitry. Radiator

[0014] is heated by combustion of incoming fuel applied to fuel supply ring

[0016] which then feeds fuel injector tubes

[0019] in its combustion chamber [8], Such injector tubes are kept cool until reaching the flame area with a small coaxial cooling airflow.

[0111] An oxidizer (such as air) may be supplied via main manifold [100a] connected to the main manifold mount surface

[0024] (as illustrated, for example, in FIGS. 1 A and 4 A and 4D through 4G) through a preheating heat exchanger

[0020] (for example, a ceramic air intake heat exchanger section) operating in countercurrent mode with the exhaust heat exchanger

[0021] The mixed oxidizer and fuel may be ignited using standard means such as a high voltage spark between fuel injector tubes

[0019] or between tubes

[0019] and radiator

[0014] , or with a catalytic igniter such as a platinum wire exposed to the flowing gas or a catalytic surface in a porous ceramic material (for example, see Moncada Quintero, et al., “Combined silicon carbide and zirconia open-cell foams for the process intensification of catalytic methane combustion in lean conditions: Impact on heat and mass transfer”, Chem. Eng. Jnl 429, 1 Feb 2022, 132448; doi.org / 10.1016 / j.cej.2021.132448). If using a spark, its high voltage power source may desirably be a manually operated piezoelectric source in the case of, for example, a simple portable generator; or a starting source of electric power applied to bus voltage connector

[0017] or control signal connector

[0022] with the high spark voltage generated as needed by a parasitic circuit

[0040] (see FIG. 5E) mounted on the module stack's fuel supply ring

[0016] ,

[0112] In a number of embodiments, a controlled-atmosphere adapter ring

[0015] may be positioned between fuel supply ring

[0016] and TPV cell generator module(s) [9], Controlled-atmosphere adapter ring

[0015] is connected to a controlled-atmosphere port

[0025] (see, for example, FIG. 3) mating with a main manifold

[0100] (see, for example, FIG. 4A) which includes or is in connection with a source of backfill gas. Controlled-atmosphere adapter ring

[0015] provides a radiator tube seal spaced distant from combustion chamber [8], The radiator tube is cooled along the length of controlled-atmosphere adapter ring

[0015] by the close proximity of heatsinking controlled-atmosphere adapter ring metal by radiative cooling and gas conduction cooling, as well as by the cool higher-pressure incoming oxidizer flow prior to its air preheat exchanger

[0020] , Such cooling, together with the choice of a high-temperature seal material, permits maintenance of sufficiently low pressures in controlled-atmosphere gap [7], Controlled-atmosphere sealing between TPV cell generator ring module [9] and process support rings is further provided by O-ring or metal-ring seals (not shown) which are positionable in sealing ring grooves

[0018] in each of the stacked ring modules and in a coolant return manifold

[0011] (see, FIG. IB). Such a gas sealing engagement may be attained in other ways; what is shown is representative of the art.

[0113] DC bus power

[0017] is connected between all of the rings which may provide initial startup power and thereafter accepts power from active ring modules [9], In the illustrated embodiments, a bidirectional control signal connection

[0022] which may be an alternative startup power source passes through each ring module affording individual control, subsidiary power, and feedback on ring module status, as needed. Such a separate control connection with its added complexity and cost may be eliminated if signals are imposed upon the DC bus line or obtained through wireless or optical connection, all of which are known in the art.

[0114] The cell voltages of a TPV cell generator ring module [9] connect to DC bus

[0017] with an electronic boost circuit

[0028] (see, for example, FIGS. IB, and 3), either from individual cells or from some series combination of cells

[0012] in that module [9], High ‘bus’ voltages are more suitable for further high-voltage interconnection, power conversion, and transmission than low cell voltages, being at a higher voltage and lower current, with consequent lower resistive connection losses. A representative example of such boost circuit

[0028] is shown in FIG. 7. The embodiment of FIG. 7 illustrates use of a single switch device, for example, an N-channel Field Effect Transistor (FET) Q, driven in an oscillatory manner from a Maximum Power-Point Tracking (MPPT) circuit

[0029] which provides a Pulse Width Modulated (PWM) drive signal to the FET. In a number of embodiments, the cell boost electronics desirably uses MPPT control to ensure that each attached cell grouping of one or more series- or parallel-connected cells [9] is operated at its maximum output power, using an oscillatory system to assure that a movement in either direction away from the chosen boost converter output power will result in a lower boost circuit output power. Such an MPPT control may, for example, desirably use the oscillatory boost circuit input current waveform to provide such oscillatory movement and power measurements concurrently with its normal boost operation. In the illustrated embodiment of FIG. 7, only one inductor L, diode D, and output bus capacitor C is needed for the illustrated minimal circuit. Alternative topologies may yield higher efficiency, especially at higher boost voltages, such as the 'flying capacitor' type, but which uses double (or more) of components Q, D and C. Many variants of boost circuits are available and any of these could be used. See, for example, M Forouzesh et al, "Review of voltage-boosting techniques, topologies, and applications", IEEE Trans. Power Electronics 32 (12) 2017 pp 9143-9178; DOI 10.1109 / TPEL.2017.2652318 . The bus-connected rectifier diode would in many embodiments be a further switched FET device to attain lower voltage drop than is attainable from a standard rectifier diode.

[0115] Bus voltage is sensed by power control

[0031] to maintain voltage within desired limits. Short-term regulation may also be able to be applied to the boost circuits. In the case of a fuel-driven system, this would, for example, be a ‘flame control’ as shown in FIG. 7. In the case of a thermal battery application such a control may, for example, mechanically move the TPV generator between positions of high and low radiant flux. Such changes can be made in far less time than the approx. 3 minutes required by high-performance natural gas turbogenerators, promising a fast response to transient demands from TPV systems hereof.

[0116] Boosting 600-700mV cell voltages to high voltages causes residual capacitive losses at FET Q's drain D connection, which can be minimized using long 'on' times and large inductors L. However, the illustrated 600V bus is not optimal for other reasons. For best efficiency at least a two-stage boost is desirable. In a two-stage boost, voltage is first boosted to a first or intermediate voltage (for example, 25 V), and then to a final bus for a desired bus voltage of 600-1000V. Boost efficiency is limited by switch device ‘on’ resistance and maximum voltage tolerance, switching times, and drain-connected capacitances. A 600V bus could use ‘superjunction’ 650V FETs, but in compact packages their ~10mQ ‘on’ resistance yields unnecessarily large losses in converting currents as low as 5A from a 0.6-0.7V cell; at 5A such a FET would exhibit a forward voltage drop of 50m V, degrading a 700mV cell output by 7%. A more efficient approach would boost to an intermediate bus voltage, with a later boost to the desired final bus voltage. For example silicon 30V 300A 400pQ ‘on’ resistance devices could boost to a 25V intermediate bus level, which higher-voltage devices could then efficiently further boost to higher bus voltages.

[0117] A further advantage of such a multi -bus scheme may be the ability to limit boost converter component count. The simple boost converter schematic illustrated above works best in the ‘continuous conduction’ mode, wherein a roughly constant current is drawn from TPV generating cell

[0012] , and no current-smoothing energy storage capacitor in parallel with TPV cells

[0012] is needed. Boost inductor energy storage thus has the potential to reduce or replace capacitor energy storage in parallel with TPV cell

[0012] , That ‘continuous conduction’ mode entails FET ‘Q’ turn-on switching losses which rise with output boost voltage, so a low intermediate bus voltage may yield a smaller component count and low overall power losses. For lowest losses, one additionally may use a synchronous rectifier FET instead of diode D to feed the low voltage bus, eliminating the ~500mV Schottky diode drop and thus its 2% efficiency hit at a 25V intermediate bus output voltage. The choice betweencontinuous- or discontinuous-conduction modes may, for example, be determined by available space, cost, and performance considerations; either mode can provide similarly high efficiency.

[0118] A series-connection approach may be used if cell-to-cell variation is small and there are equal radiation levels on each cell. Instead of boosting just one cell at a time, with cylindrically symmetric radiation in the embodiment illustrated in FIGS. 1A through 4 J, radiation flux likely varies along the length of a main module [5a] as illustrated in Fig. 4H, but not around TPV cells

[0012] of each ring module. Thus each TPV generator ring module [9] could have its total series-connected TPV cell output boosted to the intermediate bus. For example, eight cells in each inner-octagon module ring connected in series would yield an input voltage to such a module-level boost circuit of -8*0.75 =~6V. Such a higher input voltage increases boost efficiency and reduces boost circuit count. Each series-connected cell or paralleled cells in the series string may desirably have a bypass shunt FET “perfect diode” in case of individual cell failure. While this is not an ideal optimization of each individual cell’s output, such a joint voltage boosting of multiple TPV cells experiencing similar radiant intensities has the advantage of simpler construction and smaller component count.

[0119] Efficient photon recycling is an important feature of various embodiments of systems hereof. As shown in FIG. 8, for TPV cells based on silicon (Si) PV technology which require photon energies above 1.1 eV (that is, wavelengths less than approx. 1 micron), at 1900°C only 10% of the total radiated photons are in Si's useful range for electric conversion. At 1400°C for a 1 / 3 power level, only about 3% of radiated photons are useful. Thus if the 'full power' level is 1900°C, this implies, from Equation 2, a 99% low-energy-photon reflection coefficient resulting in an overall 10% loss of system efficiency, which rises to a 25% loss at the 1 / 3 power level. Thus it is desirable to provide a short path for recycled photons to the radiator, and to provide as high a heat mirror reflectance as can be attained. A further loss is apparent with single-junction silicon cell conversion; at 1900°C approx. 15% of radiated energy going into the cell (that is, 1.5% of total radiator energy) is lost due to the wasted high photon energy fraction (that is, the difference between photon energy and l.leV). With a possible Si cell TPV conversion efficiency f of 70-80%, these losses then result in full-power system efficiency of 60-70%, and a 1 / 3-power level efficiency of 45-55%. Even that low 45-55% efficiency for a 1400°C radiator is higher than most ICE generators, so may be an acceptable tradeoff for longest possible maintenance cycles.However, for best system efficiency, a modular system hereof, in a number of embodiments, shuts down unwanted main module stacks at low load while running the remaining stacks at, or close to, full power.

[0120] The short, single-module generator illustrated in FIG. 1A exhibits end losses as a result of the gap between radiator

[0014] and controlled-atmosphere adapter ring

[0015] on the right side of the figure approximating a deep and narrow ‘black body’ absorber. Thus, gap width should be held low. On the left side of FIG. 1 A the face of coolant return manifold

[0011] similarly is a possibly absorbing gap which could be alleviated by, for example, some combination of an extremely small gap, coating its metal surface with a highly reflecting coating, by shaping radiator

[0014] on its left hand side (referring to the orientation of FIG. 1A) to yield less tendency to radiate leftwards such as with a conical structure, and placing additional TPV cells

[0012] on the inner face of coolant return manifold

[0011] together with appropriate boost electronics

[0028] (see FIG. IB). Such end losses are proportionally smaller for longer stacks such as shown in FIGS. 4H through 4J. In all cases, any non-radiator region in controlled-atmosphere gap space [7] should desirably be either TPV cells

[0012] or coated with a high-reflectance material with as much of a specular reflecting surface as possible. Narrow gaps and rough surfaces are desirably avoided. A possible gap filler between cells mounted on their module ring would be a high-temperature flexible polymer such as two-part silicone, applied in a thin smooth bead between cells, and then coated with a high-reflectance film or film stack. Silicones have good high temperature resistance and relatively low outgassing but exhibit poor thermal conductivity, so such a film should be thin and primarily used to fill gaps, rather than forming a wide fillet on cell edges. Flexible polymers are advantageous because of their low thermal cycling stresses.

[0121] TPV cells

[0012] employ a front-surface heat mirror which permits photon energies above the cell bandgap to substantially pass into the cell, while substantially reflecting back to radiator

[0014] those low photon energies which are less-usefully converted by TPV cell

[0012] , Such a heat mirror may be made from one or more of a nanostructured 'optical metasurface' metallic array, a thin film enhancing low-photon-energy reflectance, a thin film enhancing high-photon-energy transmittance, and a broad-band multilayer thin film stack for high low-photon-energy reflectance. Examples of such heat mirror designs are discussed in Horne, W.E., et al., “IR filters for TPV converter modules” AIP Conference Proceedings 358, 35-54 (1996) doi.org / 10.1063 / 1.49716; Chou, S.Y., Ding, W., “Ultrathin,high-efficiency, broad-band, omni- acceptance, organic solar cells enhanced by plasmonic cavity with subwavelength hole array”" OPTICS EXPRESS A60 14 January 2013 21, No. SI; and Ismail, F.D., et al., “Filter design using multi-bragg reflectors”, World Journal of Modelling and Simulation 8 (2012) No. 3, pp. 205-210, and in W E Horne, M D Morgan "Filter Array for Modifying Radiant Thermal Energy" US Pat 5611870 (1997). Heat mirrors hereof may further reflect photons yielding excessive high-energy losses.

[0122] In addition to such top-surface photon selectivity, the TPV cell semiconductor materials desirably exhibit high carrier lifetime of more than 10ms and employ rear-surface contacts for efficient current collection with low series resistance yielding less than 40% output power loss at full power output compared to the power attainable from the multiple of open-circuit voltage and short-circuit current (that is, a 'fill factor' of more than 40%). An aim of these carrier lifetime and rear contact specifications is to assure the internal carrier recombination lifetime losses are low, and that sufficiently thick and densely packed metallic layers are provided on the cell rear to permit low series resistance losses at the high output currents and internal carrier densities employed here. Typical photovoltaic cell efficiency, when plotted against radiant photon flux, exhibits low efficiency at low fluxes due to parallel leakage resistance in the cell and a low cell output voltage. As flux rises, efficiency rises since cell output voltage rises logarithmically with its internal carrier density, and charge carrier flow dominates parallel leakage currents. However at high incident flux, efficiency falls again due to Auger recombination and other recombination losses seen at high carrier density inside cells, and also due to series resistive loss in cell contacts and in the exterior metal traces carrying current to the load. Hence there will be a peak efficiency determined by crystal purity for internal losses, by contact design, by edge passivation, and by metal density and thickness exterior to the cell. This peak efficiency should be at flux similar to, or higher than, the expected maximum radiant flux with photon energy above the cell bandgap. The combination of these specifications desirably result in the graphed cell efficiency vs. power output curve having a peak close to or above the anticipated maximum incident high-energy photon flux.

[0123] In the devices, systems, and methods hereof, it is desirable to block long-wavelength photons with energy below the TPV bandgap from entering TPV cells

[0012] , This is in contrast with many of the TPV references previously quoted in the prior art. Semiconductors with high internal carrier density, as expected here, exhibit “free-carrierabsorption” which increases with the cube of incident wavelength, thus rising sharply at low photon energies (Horwitz, C.M., Swanson, R.M., “The optical (free carrier) absorption of a hole-electron plasma in silicon”, Solid State Electronics 23, (1980) pp 1191-1194); doi.org / 10.1016 / 0038-1101(80)90111-2. By placing the reflecting surface for low-energy photon recycling on the cell surface rather than on its back, free-carrier absorption in the cell body is eliminated. Such elimination of free-carrier loss is especially important in indirect-gap semiconductors such as Si and Ge, which require substrates perhaps 20 times thicker than direct-gap semiconductors such as GaAs in the case of normally-incident radiation. In addition, metallic reflecting surfaces placed directly on TPV back surfaces have a lower reflectance than do free-standing reflectors due to a lower optical impedance mismatch between the semiconductor cell materials and the metal reflector compared to that between our gaseous controlled-atmosphere and the metal and / or dielectric stack reflector. For this additional reason, a front-surface reflector again yields lower losses than a back-surface reflector. An additional reason for choosing a front-surface reflector is its higher total reflectance, compared with current-collection structures and their inherent gaps and lightabsorbing features on TPV cell backs.

[0124] Radiator

[0014] may, for example, be constructed from a high-temperature material such as carbon, tungsten, or the like. Radiator

[0014] may further be coated internally to resist attack from combustion gases, and externally as needed to attain high emittance at the photon energies in use and to minimize evaporative loss from radiator

[0014] , Such internal coatings resistant to combustion processes are, for example, used in turbine blade treatments. See, for example, M J Mahoney, "Thermal Barrier Coating Systems and Materials", US Pat 6117560 (2000); J. Scheibel et al, "Gas Turbine Low Conductivity Thermal Barrier Coating Validation and Demonstration", 8thInternational Gas Turbine Conference 12-13 Oct 2016, Brussels; Paper ID Number 48-IGTC16. External tantalum, niobium, or hafnium carbide and other coatings to minimize evaporation from carbon are also known. See, for example, Daniel L. Deadmore, "Vaporization of Tantalum-Carbide — Hafnium-Carbide Solid Solutions at 2500° to 3000°K" NASA Technical Note TN D-2512 (1964), and E K Storms, “The Refractory Carbides”, Academic Press NY & London, (1967).

[0125] Over a long period radiator coatings facing TPV cells will oxidize and / or evaporate, thereby clouding the receiving TPV cells

[0012] , As shown in FIG. 7, tungsten has a 10,000 - 100,000 times lower evaporation rate than carbon. Carbon, however, may be a moreeconomical radiator. In that regard, in trace reactive oxygen, carbon’s transparent reaction product is CO2 which may be pumped out while consuming a thin surface layer of the radiator tube. In vacuum, if tungsten were used and operated mainly in idle mode (1400°C, 1 / 3 max. electrical output power level), in one year, and in the absence of oxidation, tungsten will evaporate only 0.0003nm, which will have no effect. However running a year at full 1850°C power yields lOnm, which will certainly increase system losses. A solution to this problem can, for example, include using of a backfill with inert gas if constant full power is required. Screening gas between radiator and TPV cells causes metal-gas collisions in a partial vacuum, forming clouds of non-adhering particle chains, rendered transparent especially at longer wavelengths in trace oxygen. See, for example, David R. McKenzie, Gold black and gold cermet absorbing surfaces, Gold Bulletin 11, 49-53 (1978); doi.org / 10.1007 / BF03215450. Such “smoke” coatings, if they form, are easily brushed or blown off. Thus, controlled-atmosphere gap [7] between radiator

[0014] and TPV cells

[0012] could be at a pressure and of a chemistry such that evaporated species from the radiator minimally occlude the cell surface.

[0126] However low-pressure fill may be less practical than using atmospheric-pressure fill with suitable purging and phosphorus-gettering or other gettering as used in the incandescent lamp industry to assure sufficient gas purity. Experiments with tungsten lamp filaments have shown that, at their much higher temperatures of 2570°C, the evaporation rate is approximately 30 times less in an atmospheric-pressure fill of 86% argon, 14% nitrogen than in vacuum. See for example G. R. Fonda, "Evaporation Rate of Tungsten under Various Pressures of Argon" Phys. Rev. 31, Feb. 1928 p 260-6; and W. Geiss, "Improvements in the Efficiency of Electric Incandescent Lamps" Philips Tech. Rev. 6, no. 11, (Nov. 1941) p334-342. This observation is thought to be due to a reduction of tungsten diffusion distance and its return and reincorporation into the hot tungsten. The nitrogen partial pressure is added in such lamps to enhance arcing breakdown voltage. In the devices, systems and methods hereof pure Ar would be equally suitable unless high voltages are required; for example if corona discharges with reactive ozone species are used to form transparent gaseous evaporant byproducts. Such gas fill may even permit the use of radiator materials such as uncoated carbon, due to its significant suppression of evaporation.

[0127] A backfill gas yields heat conduction and convection heat losses which should form an acceptably small fraction of the generated electrical power. As shown in FIG. 10, Xegas thermal conductivity at atmospheric pressure extrapolated to 2000°C is 30mW / (m-K), for a conduction of 0.6W in a Isq.cm area through a 1cm gap, compared to a typical TPV cell maximum output of 6 - lOW / sq cm (that is, 5-10% of output electric power). The heat loss is inversely proportional to the gap width, so could be reduced with >lcm radiator-cell spacing. The heat transfer rate is affected by gas rarefaction around the hot radiator which reduces convective heat losses, as considered below. In addition it should be noted that heat conductivity is -3 times lower at low temperature than at high temperature, and that the above heat loss estimate assumes the same high conductivity throughout the radiator-TPV cell gap. Hence the above heat loss number is a conservative over-estimate.

[0128] Convection increases thermal transfer, so radiator ridges to isolate convective cells may be employed. However for values of Grashof number less than 2000 - 8000 convection is suppressed and basic thermal conduction loss is the dominant loss mechanism. The Grashof number [Gr] is a measure of gas buoyancy divided by viscosity. For an ideal gas, assuming the radiator temperature T controls density and temperature difference, Gr is;3 2

[0129] Gr = (grv / T) (T g ) / v Equation 3where grv = acceleration of gravity, g = gap width, and v = kinematic viscosity. See, for example, Holman, J. P., “Heat Transfer,” 2ndEdn. McGraw-Hill 1968 p 192, 201; and Keith, F. and Bohn, M. S., “Principles of Heat Transfer”, West Publishing Co. 1993 p. 335.

[0130] The Grashof number at atmospheric pressure is approx. 200 for g = 10mm and 2000 for g = 20mm. Thus convective losses are minimal, and a large 20mm gap between TPV cells and the radiator may be tolerated, permitting conductive heat losses to also be minimized provided that photon recycling is efficient. Early studies made for the large temperature differences employed herein indicate that a ‘stagnant layer’ exists around a hot radiating wire object, indicating again gas rarefaction and lowered density around a hot radiator. Langmuir, I., “The Convection and Conduction of Heat in Gases,” Proc. 29thAnnual Convention of Amer. Inst. Of Electrical Engineers, plOl 1-1022 (June 25, 1912)].

[0131] Occasional gas purging may, for example, be performed using a small vacuum pump such as a diaphragm pump to attain short purging times, or with a purging gas flow for a period sufficient to attain the desired low oxygen concentration. If the reduction of evaporative losses with an atmospheric pressure fill is not important - for example if atungsten radiator is operated at less than its full-power temperature for most of the time -operation using that small pump to attain reduced pressure yields lower conduction loss, hence higher system efficiency.

[0132] The less expensive Ar gas could be used, though with 3x the Xe thermal conductivity at high pressures, as shown in FIG. 10. The technology of incandescent light bulb inert gas fill is well understood See, for example, Ropp, R.C., The Chemistry of Artificial Lighting Devices, Elsevier Science, New York (1993). Such technology can be applied here, with the proviso that, in contrast with incandescent light bulbs with hermetic glass-metal seals, higher permeation and gas generation rates apply in this system with adhesive bonds, sealing rings, and a high-temperature radiator with external combustion pressures, requiring a degree of periodic active gas purging.

[0133] The modular sealing-ring system described herein cannot be easily hermetically sealed with all-metal and all-glass / ceramic construction. Thus, the system cannot be permanently sealed with an appropriate gas fill. If long-life operation is required without gas refill operations, an all-metal sealed modular system may be used. Such a system may, for example, replace O-rings with compressible and deformable metal seals, and with the resultant low permeation rates be able to run for long periods without any type of maintenance. Such maintenance periods may be able to be extended with the use of gettering materials such as phosphorus as employed in the incandescent lamp industry. Other low-maintenance gettering systems such as, for example, metallic flash coatings as used in electronic vacuum tubes, getter pumps, and titanium sublimation pumps in the gas fill line could also be used and would additionally reduce the need for high-purity gas fill.

[0134] Inert gas purge plumbing may require, for example, a 3-way gas supply and pumping valve, a pressure sensor and pump control, and a small inert gas supply tank; or for simple unpumped purging, a gas supply tank, valves on inlet and outlet lines, and a flow restrictor.

[0135] TPV cell cooling is a further requirement for efficient system operation, since cell output falls with rising cell temperature. Referring for example to FIG. 3, coolant flow enters the ring structure at coolant inlets

[0027] and exits after merging flows in coolant return manifold

[0011] via coolant exit or outlet ports

[0026] , Such a flow path assists in achieving minimal countercurrent heat exchange between incoming cool fluid and outgoing heatedfluid, since the flows are substantially mechanically separate apart from the thin metallic ribs supporting the coolant exit ports

[0026] , While coolant fluid flows from coolant inlet

[0027] through a flow path passing through each ring module [9], the coolant returned to an external heat exchanger via coolant exit port

[0026] may pass through a conduit (for example, a pipe) positioned exterior to ring modules [9], and need not be structurally connected to each of the ring modules [9] (but only to return manifold

[0011] and coolant exit port

[0026] ). Such coolant flows may not be required however, in the case of substantial airflow past main modules and ring modules [9], For example, main modules [5a] shown in FIGS. 4H through 4J are shown with heatsinking fins [13’] on each ring module, which may be cooled convectively with fans or by motion through the air with 'ram air flow' in the case of mobile electricity generation.

[0136] Main modules [5a] hereof may, for example, be connected to a main manifold as described above with connections for electronic circuitry (electric output, control signals, etc.) and coolant flow. In the case of a fuel-driven generator such a main manifold in Fig. 5B [100a] would also have connections in common with other main manifolds to input oxidizer, fuel, and combustion exhaust. Many individual TPV main modules [5a] such as shown in FIGS. 4H through 4J may be mounted on several such main manifolds [100a] all connected in series, permitting easy tailoring of power level and simple maintenance. Each main manifold may advantageously incorporate valving such that removal of one TPV main module stack during operation would not interfere with operation of the remainder. Such valving may include simple sprung vanes to occlude oxidizer and exhaust flows, and more standard quick-connect fittings for fuel, controlled-atmosphere, and coolant connections.

[0137] An example of such a main manifold connection is shown in FIGS 5A through 6H, for the simple case of a single ring module forming the basis of a main module [5], In the illustrated embodiment of FIGS. 5 A through 6H, a manifold section or module [100a] includes three segments [110a], [140a], and [160a] (formed from a metal, ceramic, or other supporting material). Manifold sections [100a] may be connected in the illustrated orientation of FIG. 5A to a vertically extending TPV generator main module [5] or main module [5a], The first or upper segment [110a] (in the illustrated orientation of FIG. 5 A through 6H) contains coolant, power, and signal connections and these connect to neighboring first segments [110a] laterally (see FIG. 6H), such that a row of such first segments [110a] can control, cool, and obtain power from multiple TPV main modules [5 or 5a], Each first manifold segment [110a] may include a fuel supply pathway or conduit [112a] connected to afuel control valve [114a], permitting main module stack output power control. Valve [114a] may be off or on; or may provide a degree of proportional control where operation at reduced power may be desired, using a proportional valve as known in the flow control arts. In the illustrated embodiment, first manifold segment [110a] further includes a controlled atmosphere pathway or conduit [116a], a coolant supply pathway or conduit [120a], a coolant return pathway or conduit [122a], and power and control interconnections [130a],

[0138] For a fuel-driven main module TPV stack, a second segment [140a] underneath first segment [110a] (in the illustrated orientation) may, for example, be used to deliver oxidizer to the combustion chamber. Such oxidizer (typically air) may be delivered to combustion chamber [8] through a heat exchanger system. In the embodiment illustrated in FIGS. 5A through 5G, air is drawn in by a centrifugal compressor fan [142a] which may, for example, be driven parasitically from TPV bus electric power. As illustrated in FIG. 5E, for example, the air drawn in via centrifugal compressor fan [142a] travels in a heat exchange system formed by second segment [140a] and third segment [160a], First, the air through a scroll-shaped pattern heat exchanger [144a] designed for countercurrent flow heat exchange with exhaust gases (which pass through exhaust gas tube

[0010] ) through a thin metallic membrane or separator in Fig. 5E, 5F [146a] from a matching scroll pattern heat exchanger [162a] (see FIG. 5F) in third segment [160a], which is an exhaust gas segment. Upon reaching the central countercurrent ceramic countercurrent heat exchanger segments [20 and 21] the higher gas temperatures are then further exchanged, leading towards combustion chamber [8], Again, all manifold segments [140a and 160a] are modular, in the illustrated embodiment, and stack under (in the orientation of FIGS. 5A through 5G) first manifold segment [110a], It is evident that a horizontal line of such manifolds 100a carrying TPV main modules [5, 5a] can be parallelled with further such horizontal lines of manifolds to form a main manifold as illustrated schematically in FIG. 4A, (see, for example, FIG. 6H, illustrating two of first segments 110a aligned for connection) and a plurality of main modules [5, 5a] may be connected thereto, yielding easy system configuration to any desired power level, and simple module maintenance. The above is a simple illustrative heat exchanger design; other countercurrent heat exchanger designs are well known in the industry and can provide high heat transfer efficiency.

[0139] The above description, while applicable to fuel-burning systems, can with straightforward changes also be applied to a thermal battery where high-temperature objectsradiate as needed to a TPV collection apparatus. The ring module construction hereof, in that case, would be inverted, with TPV cells [12a] on the outside of heatsink [13a] metalwork as shown in FIG. 11. The required cooling channels and electronics are positioned inside the ring. Eight TPV cells [12a] are shown in FIG. 11, and four boost electronics systems [28a] are shown, indicating in the illustrated embodiment that two TPV cells are connected in series or parallel to each boost electronics component [28a], Other configurations and quantities of TPV cells and electronic boost components in each TPV generator ring module are also possible, dependent on TPV cell and radiation uniformity and on boost electronics capabilities.

[0140] Such an inverted arrangement can stack rings in the same manner as described above in the manner of FIGS. 4H through 4 J, yielding modular construction with fewer required connections than illustrated in FIGS 1A through 4J, since the internal radiator and pipework for controlled atmosphere, oxidizer gas, fuel, and exhaust may not be needed in the case of a thermal battery embodiment. The heated thermal storage media surrounds the TPV generator when output power is required. The response time of such a system to load changes is limited only by the rate of movement of the TPV generator into the hot radiating zone, since TPV cell and boost electronics response time is less than 1ms. Stacked TPV generator ring modules [9a] include a coolant return manifold (not shown) similar to ring module [9] at their remote end, with its attendant return pipework feeding up the center of TPV generator main module [9a], but require only a main manifold connection (not shown), without needing a controlled-atmosphere adapter ring

[0015] , fuel supply ring

[0016] , their attendant radiator

[0014] , or combustion gas components. Embodiments of a TPV thermal battery generator hereof may, however, include an intermediate radiator shield to prevent thermal storage element evaporation onto TPV cell surfaces. In such a case, a controlled-atmosphere system similar to that for a fuel-burning system would be required.

[0141] In the case of a fuel-driven generator the exhaust hot combustion products may be used as in reciprocating ICE's to drive a turbocharger, enhancing input oxidizer pressure and overall combustion flow. Such turbochargers would be used in higher-power applications which can provide sufficient high-temperature exhaust flow, but require more complex manifolding than is shown in main modular systems in of FIGS. 4A through 6H. In addition such turbochargers typically require oil pumping and cooling, and their high speeds add to reliability concerns for lengthy unattended full-power operation.

[0142] Alternatively to use of a turbocharger, parasitic power drawn from an output voltage bus may run an oxidizer pressurization such as fan [142a] on the main manifold in similarity with ICE superchargers. This has advantages of lower maintenance and drive requirements, and simpler manifolding, than a turbocharger. In addition, the systems described here are not necessarily at high pressure. The maximum internal pressure is controlled by heat exchanger pressure drops and may be of the order of 1 / 20 bar (5kPa).

[0143] While unwanted radiation reflected from TPV Cells [12a] in the case of a thermal battery (FIG. 11) will be directly returned with one reflection from TPV cell surfaces to the outer hot radiating materials, such high efficiency of radiation return is not guaranteed in the representative fuel-driven embodiments of FIGS. 1A through 5B. An illustrative embodiment of a cross-sectional geometry of FIG. 12 shows a central radiator with inner circular bore and outer octagonal geometry with a face half-width ‘d’, surrounded by an octagonal set of TPV cells facing inwards, and separated by a gap ‘g’ . Assuming Lambertian (cosine) emission from the radiator flat face, some low-energy photons will be emitted from the radiator, bounce from a TPV cell face, and then be sent through gap ‘g’ to neighboring regions, thus requiring more than one TPV cell surface reflection before being returned to the radiator for recycling. The 2-D simulated results shown in FIG. 13 indicate that as the ratio g / d increases, the chance of a single TPV cell reflection for radiation emitted at random angles from the radiator across its surface ranges from 0.93 for gap size l / 40thof the full radiator flat width, to 0.65 for gap size approx. l / 5thof the full radiator flat width.

[0144] This increased probability of more than one reflection event results in the losses shown in FIG. 14, where it is assumed that at most one extra reflection occurs if the first reflection misses the radiator. For a basic heat mirror reflection loss of 1%, when the gap is approx. l / 5thof the full radiator flat width the effective system loss becomes 1.3%. For a basic heat mirror loss of 3%, the penalty at that same gap is 1%, for an effective total reflection loss of 4%. Similar results pertain to other simple shapes. Such losses should be held to the lowest possible level to maintain system efficiency, since they are multiplied by the factor (1 / f) from Equation 2. Since however a small gap is incompatible with low inert gas fill heat conduction losses, small gaps may be inconsistent with high overall system efficiency for small ring module diameters in the presence of inert fill gas.

[0145] Heat mirror reflectance may thus be assisted with geometry such as small gaps ‘g’ relative to radiator size ‘d,’ and if that is not possible due, for example, to geometric,materials, or thermal loss considerations with a backfilled inert gas, assisted with structures such as shown in FIGS. 15A and 15B. As illustrated, gap 'g' in TPV generator module [9b] is narrowed between TPV cells with a radiator extension

[0032] which extend from a surface of radiator 14b. An alternative triangular-section TPV generator module extension of cell heatsinking material between cells would similarly limit photon losses but would require a highly reflective extension between cells, with its attendant reflection loss, so radiator extensions are preferable when mechanically feasible. Such extensions permit larger gaps 'g', hence low conduction loss in backfill inert gases, since gap size 'g' may become arbitrarily large without compromising photon recycling efficiency.

[0146] The foregoing description and accompanying drawings set forth a number of representative embodiments at the present time. Various modifications, additions and alternative designs will, of course, become apparent to those skilled in the art in light of the foregoing teachings without departing from the scope hereof, which is indicated by the following claims rather than by the foregoing description. All changes and variations that fall within the meaning and range of equivalency of the claims are to be embraced within their scope.

Claims

WHAT IS CLAIMED IS:

1. A thermophotovoltaic (TPV) generator, comprising:one or more ring modules comprising a frame formed from a high thermal conductivity material formed as a ring, each of the one or more ring modules being adapted on each axial end thereof to be attached to another of the one or more ring modules in an axially stacked manner with a gas sealing engagement between them, each of the one or more ring modules comprising a plurality of TPV cells, which have a TPV cell output power no lower than 40% of the product of open circuit voltage and short circuit current at the maximum expected incident photon flux with energies above a TPV cell bandgap, attached on either a radially inner surface of the ring or on a radially outer surface of the ring, each of the TPV cells being configured to convert photons with energy above the TPV cell bandgap thereof from a radiator into electrical energy, each of the TPV cells comprising a heat mirror on a surface thereof, facing the radiator, which reflects photons with an energy lower than the TPV cell bandgap back to the radiator, wherein the radiator is positionable within a central volume of the one or more ring modules when the plurality of TPV cells are attached on the radially inner surface of the ring or positioned radially outside of a perimeter of the one or more ring modules when the plurality of TPV cells are attached on the radially outer surface of the ring such that the TPV cells are oriented to face a radiating surface of the radiator, and wherein the TPV cells are mounted on polygonal surfaces of the ring facing the radiator.

2. The TPV generator of claim 1 wherein the number of ring modules attached in the axially stacked manner is determined based upon a predetermined maximum power level for the stacked ring modules forming a main module.

3. The TPV generator of claim 2 wherein the plurality of TPV cells face toward the radiating surface of the radiator with a geometry such that photons reflected from a surface of each of the plurality of TPV cells have a probability of failure to directly return low-energy photons to the radiator no greater than double the TPV cell low-energy photon reflectance loss.

4. The TPV generator of claim 2 wherein gaps between TPV cells include a reflective coating such that an overall ring module and main module reflectance loss is no greater than double the TPV cell reflectance loss for photon energies lower than the TPV cell bandgap.

5. The TPV generator of claim 2 wherein each of the one or more ring modules comprises a DC bus segment, a control system, a communication system, and circuitry in connection with the DC bus segment and the communication system, the circuitry being configured to boost outputs of the plurality of TPV cells of the ring modules to a higher de bus level to be output from the TPV generator, wherein the DC bus segment of each of the one or more ring modules is placed in electrical connection with the DC bus segment of each of another of the one or more ring modules when attached in the stacked manner to form the main module.

6. The TPV generator of claim 5 wherein the communication system comprises a control line segment, and the control line segment of each of the one or more ring modules is placed in electrical connection with the control line segment of each of another of the one or more ring modules when attached in the stacked manner.

7. The TPV generator of claim 5 wherein the communication system of each of the one or more ring modules is configured to place the ring module in communication with a central electronic circuitry comprising a central control system in a wired or wireless manner.

8. The TPV generator of claim 5 comprising a plurality of the ring modules which are attached in the stacked manner to form the main module and DC bus power output from the TPV generator is fed by one or more of the plurality of ring modules to a main module DC bus output using one or more DC bus levels to attain a final high de bus voltage.

9. The TPV generator of claim 5 comprising a plurality of main modules and wherein each of the plurality of main modules is controlled via signals communicated between the communication systems of each of the plurality of ring modules of each of the plurality of main modules and a central electronic circuitry comprising a central control system in a wired or wireless manner.

10. The TPV generator of any one of claims 1 through 9 wherein the TPV cells of each of the one or more ring modules are attached on the radially outer surface of the ring module, each of the one or more ring modules further comprising circuitry configured to boost the outputs of the plurality of TPV cells and for control of the ring module, the circuitry being positioned on an inside of the ring, the generator being used to generate electric power from heat radiated from the radiator to operate as a thermal battery.

11. The TPV generator of claim 10 wherein each of the one or more ring modules comprises a coolant fluid conduit, the TPV generator further comprising a coolant return conduit via which a coolant fluid which has passed through the coolant fluid conduit in each of the one or more ring modules is returned to an external heat exchanger.

12. The TPV generator of claim 11 further comprising a coolant return manifold in connection with a distal end ring module of the one or more ring modules and comprising a conduit to receive the coolant fluid from the coolant fluid conduit of the distal end ring module and return the coolant fluid to the external heat exchanger via the coolant return conduit.

13. The TPV generator of any one of claims 1 through 9 wherein the TPV cells of each of the one or more ring modules are attached on the radially inner surface of the ring, each of the one or more ring modules further comprising circuitry configured to boost the outputs of the plurality of TPV cells and for control of the ring module, the circuitry being positioned on an exterior of the ring, the TPV generator being used to generate electric power from heat radiated from the radiator.

14. The TPV generator of claim 13 wherein each of the one or more ring modules comprises a coolant fluid conduit, the TPV generator further comprising a coolant return conduit via which a coolant fluid which has passed through coolant fluid conduit of the one or more ring modules is returned to an external heat exchanger,15. The TPV generator of claim 14 further comprising a coolant return manifold in connection with a distal end ring module of the one or more ring modules and comprising a conduit to receive the coolant fluid from the coolant fluid conduit of the distal end ring module and return the coolant fluid to the external heat exchanger via the coolant return conduit.

16. The TPV generator of any one of claims 1 through 9 where the radiator is surrounded by a fill gas, wherein the fill gas is at a controlled pressure, and wherein the fill gas assists in radiator evaporation suppression and in suppression of radiator degradation.

17. The TPV generator of claim 16 where the fill gas is inert.

18. The TPV generator of claim 10 wherein a sealing engagement is created between each of the one or more ring modules when axially stacked which provides a seal to maintain a controlled atmosphere between the radiator and the plurality of TPV cells of each of the one or more ring modules.

19. The TPV generator of claim 13 where the radiator is heated by fuel combustion and wherein oxidizer gas supplied to the radiator is preheated by heat exchange with exhaust gas from combustion of the fuel.

20. The TPV generator of claim 19 wherein the oxidizer gas is supplied to a combustion chamber of the radiator and is pressurized with a fan powered either from the DC bus power of the TPV generator or from the exhaust gas using a turbocharger .

21. The TPV generator of any one of claims 1 through 9 further comprising a controlled-atmosphere ring attachable to one of the one or more ring modules, and via which a controlled-atmosphere can be introduced to a central volume.

22. The TPV generator of any one of claims 1 through 9 further comprising a fuel supply ring via which fuel is supplied to the radiator.

23. The TPV generator of claims 1 through 9 wherein the main module is attachable to a main manifold which can accept a plurality of main modules.

24. The TPV generator of claim 22 wherein the main module is attachable to a main manifold which can accept a plurality of main modules.

25. The TPV generator of any one of claim 1 through 9 wherein the high thermal conductivity material has a thermal conductivity of no less than 100W / (m-K).

26. The TPV generator of any one of claim 1 through 9 wherein the heat mirror comprises a metallic nanostructured heat mirror or broad-band multilayer dielectric stack heat mirror.

27. The TPV generator of any one of claim 1 through 9 wherein the heat mirror further reflects photons that would yield above-bandgap high-energy losses.

28. A TPV generator, comprising: a plurality of main modules, each of the main modules comprising one or more ring modules comprising a frame formed from a highthermal conductivity material formed as a ring, each of the one or more ring modules being adapted on each axial end thereof to be attached to another of the one or more ring modules in an axially stacked manner with a gas sealing engagement between them, each of the one or more ring modules comprising a plurality of TPV cells, which have a TPV cell output power no lower than 40% of the product of open circuit voltage and short circuit current at the maximum expected incident photon flux with energies above a TPV cell bandgap, attached on either a radially inner surface of the ring or on a radially outer surface of the ring, each of the TPV cells being configured to convert photons with energy above the TPV cell bandgap thereof from a radiator into electrical energy, each of the TPV cells comprising a heat mirror on a surface thereof, facing the radiator, which reflects photons with an energy lower than the TPV cell bandgap back to the radiator and optionally reflects higher-energy photons that would exhibit excessive energy losses inside the TPV cell, wherein the radiator is positionable within a central volume of the one or more ring modules when the plurality of TPV cells are attached on the radially inner surface of the ring or positioned radially outside of a perimeter of the one or more ring modules when the plurality of TPV cells are attached on the radially outer surface of the ring such that the TPV cells are oriented to face a radiating surface of the radiator, and wherein the TPV cells are mounted on polygonal surfaces of the ring facing the radiator, wherein the number of ring modules attached in the axially stacked manner is determined based upon a predetermined maximum power level for the stacked ring modules forming each of the plurality of main modules.

29. The TPV generator of claim 28 wherein the number of ring modules attached in the axially stacked manner is determined based upon a predetermined maximum power level for the stacked ring modules forming each of the plurality of main modules.

30. The TPV generator of claim 29 wherein the plurality of TPV cells face toward the radiating surface of the radiator with a geometry such that photons reflected from a surface of each of the plurality of TPV cells have a probability of failure to directly return low-energy photons to the radiator no greater than double the TPV cell low-energy photon reflectance loss.

31. The TPV generator of claim 29 wherein gaps between TPV cells include a reflective coating such that an overall ring module and main module reflectance loss is no greater than double the TPV cell reflectance loss for photon energies lower than the TPV cell bandgap.

32. The TPV generator of claim 29 wherein each of the one or more ring modules comprises a DC bus segment, a control system, a communication system, and circuitry in connection with the DC bus segment and the communication system, the circuitry being configured to boost outputs of the plurality of TPV cells of the ring modules to a higher de bus level to be output from the TPV generator, wherein the DC bus segment of each of the one or more ring modules is placed in electrical connection with the DC bus segment of each of another of the one or more ring modules when attached in the stacked manner to form the main module.

33. The TPV generator of claim 32 wherein the communication system comprises a control line segment, and the control line segment of each of the one or more ring modules is placed in electrical connection with the control line segment of each of another of the one or more ring modules when attached in the stacked manner.

34. The TPV generator of claim 32 wherein the communication system of each of the one or more ring modules is configured to place the ring module in communication with a central electronic circuitry comprising a central control system in a wired or wireless manner.

35. The TPV generator of claim 32 comprising a plurality of the ring modules which are attached in the stacked manner to form each of the plurality of main modules and DC bus power output from the TPV generator is fed by one or more of the plurality of ring modules to a main module DC bus output of each of the plurality of main modules using one or more DC bus levels to attain a final high de bus voltage.

36. The TPV generator of claim 32 wherein each of the plurality of main modules is controlled via signals communicated between the communication systems of each of the plurality of ring modules of each of the plurality of main modules and a central electronic circuitry comprising a central control system in a wired or wireless manner.

37. The TPV generator of any one of claims 28 through 36wherein each of the main modules comprises a fuel supply ring via which fuel is supplied to the radiator thereof and wherein the fuel supply ring is attached to a main manifold.

38. The TPV generator of claim 37 further comprising a fuel supply system in fluid connection with the main manifold.

39. The TPV generator of claim 38 further comprising at least a second main manifold, the second main manifold independently having a plurality of main modules.

40. A method of generating electricity, comprising: providing a TPV generator as set forth in any one of claims 1 through 9.

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