Metamaterial Enhanced Thermophotovoltaic Converter

Abstract

A thermophotovoltaic (TPV) converter includes a spectrally-selective metamaterial emitter and an associated “matched” photovoltaic (PV) cell. The PV cell has an optimal conversion spectrum (i.e., the wavelength range of in-band photons efficiently converted into electricity). The metamaterial emitter is fabricated with bull's eye (circular target-shaped) structures made up of concentric circular ridges that are set at a fixed grating period roughly equal to the associated optimal conversion spectrum. When the emitter is heated to a high temperature (i.e., above 1000° K), thermally excited surface plasmons generated on the concentric circular ridges produce a highly-directional radiant energy beam having a peak emission wavelength that is roughly equal to the fixed grating period and is directed onto the PV cell. The metamaterial emitter is optionally provided with multiple bull's eye structures in a multiplexed (overlapping) pattern and with different grating periods to produce a broad emission spectrum overlapping the optimal conversion spectrum.

1 . A thermophotovoltaic (TPV) converter comprising:
 a spectrally-selective metamaterial emitter including one or more bull's eye structures disposed on a solid base substrate, each of said one or more bull's eye structures including a plurality of concentric circular ridge structures separated by intervening circular grooves such that each adjacent pair of ridge structures is separated by a fixed grating period; and   a photovoltaic (PV) cell fixedly disposed to receive radiant energy generated by said spectrally-selective metamaterial emitter, said PV cell including means for converting in-band photons at a higher efficiency than out-of-band photons, said in-band photons having associated first wavelengths within an optimal conversion wavelength range, and said out-of-band photons having associated second wavelengths outside of said optimal conversion wavelength range,   wherein said one or more bull's eye structures are configured such that, when the metamaterial emitter is heated to a temperature above 1000° K, radiant energy is emitted from said one or more bull's eye structures having a peak emission wavelength that is within said optimal wavelength range, whereby said spectrally-selective metamaterial emitter is matched to the PV cell.   
     
     
         2 . The TPV converter of  claim 1 ,
 wherein the optimal wavelength range of said PV cell is 1 microns to 3 microns, and   wherein said one or more bull's eye structures are configured such that the fixed grating period separating the concentric circular ridge structures of each of said one or more bull's eye structures is in the range of 10 nanometers and 5.0 microns.   
     
     
         3 . The TPV converter of  claim 2 ,
 wherein the PV cell comprises a low-bandgap PV cell and the optimal wavelength range of said PV cell is 1.0 microns to 2.0 microns, and   wherein said one or more bull's eye structures are configured such that the fixed grating period is in the range of 0.5 microns and 3.0 microns.   
     
     
         4 . The TPV converter of  claim 3 , wherein said spectrally-selective metamaterial emitter consists entirely of metal. 
     
     
         5 . The TPV converter of  claim 1 , wherein said one or more bull's eye structure comprises a plurality of bull's eye structures disposed in sets on said base substrate, each said set comprising a first bull's eye structure including a first group of concentric circular ridge structures having a first fixed grating period, and a second bull's eye structure including a second group of concentric circular ridge structures having a second fixed grating period, and wherein the second fixed grating period is larger than the first fixed grating period. 
     
     
         6 . The TPV converter of  claim 1 ,
 wherein said one or more bull's eye structure comprises a first bull's eye structure and a second bull's eve structure disposed on said base substrate, said first bull's eye structure including a first group of concentric circular ridge structures, and said second bull's eye structure including a second group of concentric circular ridge structures, and   wherein the first and second bull's eye structures are multiplexed such that at least some of the circular ridge structures of the first group intersect at least some of the circular ridge structures of second group.   
     
     
         7 . The TPV converter of  claim 6 ,
 wherein said first group of concentric circular ridge structures of said first bull's eye structure have a first fixed grating period, and said second group of concentric circular ridge structures of said second bull's eye structure have a second fixed grating period, and   wherein the second fixed grating period is larger than the first fixed grating period.   
     
     
         8 . A TPV converter comprising:
 a metamaterial emitter including:   a box-like enclosure at least partially formed by a peripheral wall including spaced-apart first and second wall portions respectively having first and second inward-facing surfaces that faces an interior cavity of the enclosure, and respectively having first and second outward-facing surfaces that face away from the interior cavity,   at least one first bull's eye structure disposed on the first outward-facing surface of the first wall portion and including a plurality of first concentric circular ridge structures disposed in an associated first fixed grating period such that, when the metamaterial emitter is heated to a temperature above 1000° K, a first radiant energy beam is emitted from said one or more first bull's eye structures having a peak emission wavelength that is roughly equal to said first fixed grating period, and   at least one second bull's eye structure disposed on the second outward-facing surface of the second wall portion and including a plurality of second concentric circular ridge structures disposed in an associated second fixed grating period such that, when the metamaterial emitter is heated to said temperature above 1000° K, a second radiant energy beam is emitted from said one or more second bull's eye structures having a peak emission wavelength that is roughly equal to said second fixed grating period; and   first and second photovoltaic (PV) cells fixedly disposed adjacent to the spectrally-selective metamaterial emitter such that said first PV cell is positioned to receive said first radiant energy beam emitted from said first bull's eye structure, and such that said second PV cell is positioned to receive said second radiant energy beam emitted from said second bull's eye structure.   
     
     
         9 . The TPV converter of  claim 3 , wherein said box-like enclosure comprises an all-metal structure. 
     
     
         10 . The TPV converter of  claim 9 , wherein said box-like enclosure comprises one or more refractory metals. 
     
     
         11 . The TPV converter of  claim 9 ,
 wherein said box-like enclosure comprises an inlet end and outlet end,   wherein said peripheral wall includes first and second peripheral wall portions disposed in an opposing spaced-apart relationship and respectively extending between of said inlet and outlet ends of said box-like enclosure such that an inlet opening is defined between respective first end portions of said first and second peripheral wall portions, and an outlet opening is defined between respective second end portions of said first and second peripheral wall portions, and   wherein the at least one bull's eye structure includes a first bull's eye structure disposed on the first outward-facing surface of said first peripheral wall portion, and a second bull's eye structure disposed on the second outward-facing surface of said second peripheral wall portion.   
     
     
         12 . The TPV converter of  claim 9 , wherein said box-like enclosure further comprises first and second compound parabolic trough structures respectively integrally connected to the first end portions of said first and second peripheral wall portions. 
     
     
         13 . The TPV converter of  claim 12 , wherein said box-like enclosure further comprises first and second funnel-shaped outlet structures respectively integrally connected to the second end portions of said first and second peripheral wall portions. 
     
     
         14 . The TPV converter of  claim 13 , wherein the at least one bull's eye structure includes a first array of multiplexed bull's eye structures disposed on the first outward-facing surface of said first peripheral wall portion, and a second array of multiplexed bull's eye structures disposed on the second outward-facing surface said second peripheral wall portion. 
     
     
         15 . The TPV converter of  claim 14 , wherein the first and second peripheral wall portions, the first and second compound parabolic trough structures and the first and second funnel-shaped outlet structures comprise a single refractory metal. 
     
     
         16 . A method for producing a thermophotovoltaic (TPV) converter including a spectrally-selective metamaterial emitter and a photovoltaic (PV) cell, said PV cell including means for efficiently converting into electricity radiant energy that is directed onto said PV cell and has an associated optimal conversion spectrum, the method comprising:
 determining the associated optimal conversion spectrum of the PV cell; and   fabricating the spectrally-selective metamaterial emitter such that the spectrally-selective metamaterial emitter includes one or more bull's eye structures, each of said one or more bull's eye structures including a plurality of concentric circular ridge structures separated by a fixed grating period that is within the determined associated optimal conversion spectrum; and   fixedly mounting the PV cell relative to the metamaterial emitter such that, when the spectrally-selective metamaterial emitter is subsequently heated to a temperature above 1000° K, radiant energy emitted from said one or more bull's eye structures is directed onto the PV cell.   
     
     
         17 . The method of  claim 16 , wherein fabricating the metamaterial emitter comprises:
 utilizing photolithography to generate a patterned mask on a planar surface of a solid metal substrate such that the patterned mask includes a plurality of concentric circular resist structures having said fixed grating period;   utilizing the mask to form said plurality of concentric circular ridge structures on the planar surface; and   removing said mask from the planar surface of the solid metal substrate, thereby forming said one or more bull's eye structure including said plurality of concentric circular ridge structures having said fixed grating period.   
     
     
         18 . The method of  claim 17 ,
 wherein said solid metal substrate comprises a first refractory metal, and   wherein utilizing the mask to form a plurality of concentric circular ridge structures comprises one of:
 depositing a second refractory metal into the intervening concentric circular slots of said mask; and 
 etching said solid metal substrate through said intervening concentric circular slots of said mask. 
   
     
     
         19 . The method of  claim 16 , wherein fabricating the metamaterial emitter comprises forming said one or more bull's eye structures with said fixed grating period in the range of 0.5 microns and 5.0 microns. 
     
     
         20 . The method of  claim 19 ,
 wherein the PV cell comprises a low-bandgap PV cell having said optimal wavelength range of 1.0 microns to 2.0 microns, and   wherein fabricating the metamaterial emitter comprises forming a plurality of said bull's eye structures having associated said fixed grating periods in the range of 0.5 microns and 3.0 microns.