Abstract:
Systems and methods are operable to generate electric power from heat. An exemplary direct thermal electric converter embodiment includes at least a first recombination material having a first recombination rate, a second recombination material adjacent to the first recombination material and having a second recombination rate, wherein the second recombination rate is different from the first recombination rate, and a third recombination material adjacent to the second recombination material and having a third recombination rate substantially the same as the first recombination rate. Application of heat generates at least first charge carriers that migrate between the first recombination material and the second recombination material, and generates at least second charge carriers that migrate between the third recombination material and the second recombination material. The migration of the first charge carriers and the migration of the second charge carriers generates an electrical current.

Description:
PRIORITY CLAIM 
     This application is a divisional of U.S. Pat. No. 8,624,100 issued on Jan. 7, 2014, which is claims the benefit of and priority to U.S. provisional application entitled “Useful Electrical Power from Thermally Generated Carrier Pairs”, having application Ser. No. 61/381,984, filed Sep. 11, 2010, and which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Heat is a readily available source of energy. Heat may be available from ambient sources, such as the atmosphere, flowing water, the sun, or geothermal fluids. Heat may also be a byproduct of a process such as steam-powered electrical generation, or industrial manufacturing, operating semiconductor devices, or the like. 
     However, it has been difficult to convert available heat energy into electrical power. For example, heat may be used to generate power using a secondary fluid, such as steam or the like, which drives a generator turbine. 
     Often, such as at electrical power generation stations and industrial manufacturing facilities, heat is considered as a waste byproduct that must be eliminated. When heat is a waste byproduct, for example, the waste heat is dissipated into the atmosphere using cooling towers or the like. 
     Accordingly, at least to improve thermal efficiency and perhaps to reduce systems complexity, there is a need in the arts to derive electrical power from available heat. 
     SUMMARY 
     Systems and methods of direct thermal electric conversion are disclosed. An exemplary embodiment includes at least a first recombination material having a first recombination rate, a second recombination material adjacent to the first recombination material and having a second recombination rate, wherein the second recombination rate is different from the first recombination rate, and a third recombination material adjacent to the second recombination material and having a third recombination rate substantially the same as the first recombination rate. Application of heat generates at least first charge carriers that migrate between the first recombination material and the second recombination material, and generates at least second charge carriers that migrate between the third recombination material and the second recombination material. The second charge carriers are opposite in polarity from the first charge carriers. The migration of the first charge carriers and the migration of the second charge carriers generates an electrical current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred and alternative embodiments are described in detail below with reference to the following drawings: 
         FIG. 1  is a block diagram of an embodiment of a direct thermal electric converter; 
         FIG. 2  is a block diagram of an exemplary semiconductor embodiment of the direct thermal electric converter; 
         FIG. 3  conceptually illustrates migration of mobile charge carriers from the low recombination material into the positive doped layer, and then the attendant migration of holes from the positive doped layer into the high recombination material; 
         FIG. 4  conceptually illustrates migration of mobile charge carriers from the low recombination material into the negative doped layer, and then the attendant migration of electrons from the negative doped layer into the high recombination material; 
         FIG. 5  is a block diagram of an alternative semiconductor embodiment of the direct thermal electric converter; 
         FIG. 6  is a block diagram of an alternative semiconductor embodiment of the direct thermal electric converter; and 
         FIG. 7  is a block diagram of an electrochemical embodiment of the direct thermal electric converter. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of an embodiment of a direct thermal electric converter  100 . Embodiments of the direct thermal electric converter  100  are configured to receive heat from a heat source  102 , and are configured to generate electrical power from the received heat. The electrical power (current and voltage) is deliverable to a load  104 . In some embodiments, the generated electrical power is output in a direct current (DC) form. In other embodiments which include power condition equipment, the generated electrical power may be output in an alternating current (AC) form. 
     The various semiconductor embodiments are configured to capture the “built-in” potential (V D ) of diodes by pitting the depletion regions of different materials against each other. The overall formula for carrier densities in semiconductors is demonstrated in Equation 1.
 
 n*p=C*T   3   e   (−Eg/kb*T)   (1)
 
     In Equation 1, n and p are electron and hole concentrations respectively, C is a material-specific constant, T is temperature (in Kelvins), Eg is the bandgap, and kb is Boltzmann&#39;s Constant. At ambient, kb*T is approximately 0.025 eV. 
       FIG. 2  is a block diagram of an exemplary semiconductor embodiment  200  of the direct thermal electric converter  100 . The exemplary semiconductor embodiment  200  comprises alternating layers of a low recombination material  202  (interchangeably referred to as a low recombination semiconductor material) and a high recombination material  204  (interchangeably referred to as a high recombination semiconductor material). The low recombination material  202  and the high recombination material  204  join at a heterojunction. 
     The high recombination material  204  may be doped with a positive doping material to form a positive doped layer  206   a  at one end and adjacent to the heterojunction. The high recombination material  204  may doped with a negative doping material to form a negative doped layer  208   a  at the other end and adjacent to the other heterojunction. Accordingly, a layer of high recombination material  204  remains that is not doped (and is thus on opposing sides of the positive doped layer  206   a  and the negative doped layer  208   a ). 
     The low recombination material  202  may also be doped with a positive doping material to form a positive doped layer  206   b  at one end and adjacent to the heterojunction. The low recombination material  202  may also be doped with a negative doping material to form a negative doped layer  208   b  at the other end and adjacent to the another heterojunction. Accordingly, a layer of low recombination material  202  remains that is not doped. 
     As illustrated in  FIG. 2 , the high recombination material  204  and the low recombination material  202  are separated by either a positive doped layer  206   a/b  or a negative doped layer  208   a/b . Electrons may migrate across the heterojunction through the negative doped layer  208   a/b . Holes may migrate across the heterojunction through the positive doped layer  206   a/b . Electron and/or hole drift, diffusion, and thermionic emission (indicating crossing the heterojunction) may be interchangeably used for the term “migrate” herein. 
     A positive terminal  210  and a negative terminal  212  provide attachment points (a Schottky contact or the like) for delivery of the generated DC electric power. In alternative embodiments, an optional positive doped layer  214  and/or an optional negative doped layer  216  may be included at the ends of the low recombination material  202  and/or the high recombination material  204 , respectively, to provide an ohmic type contact with the terminals  210 ,  212 . 
     Any suitable low recombination material  202  and high recombination material  204  may be used. Any suitable doping material type, doping layer depth, and/or impurity concentration may be used in the various embodiments. In some embodiments, different doping materials may be used. 
     The low recombination material  202 , the high recombination material  204 , the positive doped layer  206   a/b , and the negative doped layer  208   a/b  are semiconductor type materials wherein the bandgap between the conduction band and the valence band is relatively small (as compared to an insulator type material). In the high recombination material  204 , electrons and holes may more easily recombine, or may be annihilated, as compared to the low recombination material  202 . Since the rate at which electrons and holes recombine is inversely proportional to the electron and hole concentrations in the conduction and valence bands, respectively, the low recombination material  202  has relatively more free electrons and holes at any given temperature as compared to the high recombination material  204 . Accordingly, there are a relatively greater number of free electrons and holes that are available to migrate from the low recombination material  202  (as compared to the high recombination material  204 ). Various embodiments may be created using selected materials with relatively high and low recombination rates of interest to achieve desired current and/or voltage in a semiconductor embodiment  200 . 
     When heat energy is applied or transmitted into the semiconductor materials  202 ,  204 ,  206   a/b , and/or  208   a/b , mobile charge carriers (electrons) are able to migrate from their valence band up to their respective conduction band. Once the electron migrates to its conduction band, the electron may readily move to conduction bands of adjacent atoms or molecules. The associated hole created by the electron is also a mobile charge carrier that may readily migrate to adjacent atoms or molecules. 
     The mobile charge carriers (electron) of the low recombination material  202  tend to remain in the conduction bands due to the inherent nature of the low recombination material  202  which tends to resist recombination of the electron and hole pairs. Thus, it is relatively easy for the mobile charge carriers of the low recombination material  202  to migrate to other conduction bands of adjacent atoms or molecules. 
     In contrast, the electrons and/or holes that have migrated into the high recombination material  204  tend to recombine. Electron mobile charge carriers drop out of the conduction bands into available holes of the valence bands due to the inherent nature of the high recombination material  204  which tends to facilitate recombination of the electrons and/or holes. That is, it is relatively easy for the electrons in the conduction band to recombine with the holes of the valence bands. 
     The migration of mobile charge carriers (electrons and/or holes) may be directed, guided, limited and/or constrained in a manner that induces (generates) an aggregate electric current flow and an attendant voltage in the semiconductor embodiment  200  of the direct thermal electric converter  100 . The current and voltage available at the terminals  210 ,  212  may then be provided to a load  104  ( FIG. 1 ). 
       FIG. 3  shows a portion  214  of the semiconductor embodiment  200  that conceptually illustrates migration of mobile charge carriers (electrons and holes) from the low recombination material  202  into the positive doped layer  206   a/b , and then the attendant migration of holes from the positive doped layer  206   a/b  into the high recombination material  204 .  FIG. 4  shows a portion  216  of the semiconductor embodiment  200  conceptually illustrates migration of mobile charge carriers (electrons and holes) from the low recombination material  202  into the negative doped layer  208   a/b , and then the attendant migration of electrons from the negative doped layer  208   a/b  into the high recombination material  204 . Holes are conceptually illustrated as a “o” and electrons are conceptually illustrated as an “e” in  FIGS. 3 and 4 . The holes are opposite in polarity from the electrons. 
     The positive doped layer  206   a/b  is a semiconductor layer fabricated with impurities that result in a relatively large number of holes in the positive doped layer  206   a/b . As conceptually illustrated in  FIG. 3 , when the mobile charge carriers (electrons and/or holes) from an adjacent low recombination material  202  migrate into the positive doped layer  206   a/b , the migrating electrons tend to be repelled or recombine with the holes of the positive doped layer  206   a/b , respectively. However, the migrating holes from the low recombination material  202  tend to migrate through the positive doped layer  206   a/b  into the high recombination material  204 . This net movement of holes from the low recombination material  202 , through the positive doped layer  206   a/b , and then the into the high recombination material  204  results in a generated current and voltage. 
     The negative doped layer  208   a/b  is fabricated with impurities that result in a relatively large number of electrons in the negative doped layer  208   a/b . As conceptually illustrated in  FIG. 4 , when the mobile charge carriers (electrons and/or holes) from an adjacent low recombination material  202  migrate into the negative doped layer  208   a/b , the migrating holes tend to be repelled or recombine with the electrons of the negative doped layer  208   a/b , respectively. However, the migrating electrons from the low recombination material  202  tend to migrate through the negative doped layer  208   a/b  into the high recombination material  204 . This net movement of electrons from the low recombination material  202 , through the negative doped layer  208   a/b , and then the into the high recombination material  204  results in a generated current and voltage. 
     The holes migrating through the positive doped layer  206   a/b  into the high recombination material  204  tend to combine with the electrons migrating through the negative doped layer  208   a/b  into the high recombination material  204 . As the electrons and holes recombine in the high recombination material  204 , additional mobile charge carriers (electrons and/or holes) tend to further migrate into the high recombination material  204 . The continual migration of the mobile charge carriers tends into the high recombination material  204  results in a sustainable generated current and voltage so long as sufficient heat energy is available in the low recombination material  202  to generate mobile charge carriers. and a load is present to absorb the current. If no load is present, a maximum open circuit voltage will be reached, reducing the net migration to zero. 
       FIG. 5  is a block diagram of an alternative semiconductor embodiment  500  of the direct thermal electric converter  100 . The semiconductor embodiment  500  comprises alternating layers of a high recombination material  204  and a low recombination material  202  which are separated by either a positive doped layer  206   a/b  or a negative doped layer  208   a/b . A positive terminal  210  and a negative terminal  212  provide attachment points for delivery of the generated DC electrical power. In alternative embodiments, a positive doped layer and/or a negative doped layer (not shown) may be optionally included at the ends of the semiconductor embodiment  500  depending upon the material recombination type of the ending portions of the semiconductor materials. 
       FIG. 6  is a block diagram of an alternative semiconductor embodiment  600  of the direct thermal electric converter  100 . The semiconductor embodiment  600  comprises a plurality of alternating layers of a high recombination material  204  and plurality of a low recombination material  202  which are separated by either a positive doped layer  206   a/b  or a negative doped layer  208   a/b . A positive terminal  210  and a negative terminal  212  provide attachment points for delivery of the generated DC electrical power. In alternative embodiments, a positive doped layer  214  and/or a negative doped layer  216  may be optionally included at the ends of the semiconductor embodiment  200  depending upon the material recombination type of the ending portions of the semiconductor materials. 
     The plurality of layers of alternating high recombination material  204  and low recombination material  202  permit generation of a higher voltage and/or current at the terminals  210 ,  212 . Accordingly, the design and fabrication of the semiconductor embodiment  500  may be engineered to provide any suitable voltage and/or current of interest. Either material may be used at a terminal, without regard to the material used at the opposite terminal. 
     In some applications, groups of the semiconductor embodiments  200 ,  500 ,  600  may be arranged in parallel and/or series connection configurations to further provide a voltage and/or current of interest. Thus, some semiconductor embodiments  200 ,  500 ,  600  may be configured to source low voltage and/or low current loads  400 . Other semiconductor embodiments  200 ,  500 ,  600  may be configured to source high voltage and/or high current loads  104 . Some embodiments may be configured to supplement, or even replace, power generation stations used in a public utility power grid or a private power system. Where waste heat is available, semiconductor embodiments  200 ,  500 ,  600  may be used for energy conservation, green power, and/or co-generation. 
     In a working semiconductor embodiment  600 , a three inch wafer was fabricated using molecular beam epitaxy with 21 total layers (10.5 pairs), each 0.25 um thick, alternating 0.50 AlGaAs (50% Al) and 0.33 AlGaAs (30% Al) on an n-doped GaAs wafer. The top and bottom 10% (25 nm) of each layer were doped at 1.0E+18, with alternating doping at each heterojunction. Terminals  210 ,  212  were deposited on top and bottom of the finished wafer and annealed. Table 1 illustrates measured test performance results for the working semiconductor embodiment  600 . 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Temperature (F.) 
                 Voltage (mV) 
                 Current (uA) 
               
               
                   
                   
               
             
             
               
                   
                 250 
                 0.026 
                 0.04 
               
               
                   
                 300 
                 0.051 
                 0.11 
               
               
                   
                 350 +/− 25 (cyclical) 
                   
                 0.03 to 0.72 
               
               
                   
                 450 +/− 25 (cyclical) 
                   
                 0.27 to 1.95 
               
               
                   
                 550 +/− 25 (cyclical) 
                   
                 0.40 to 3.07 
               
               
                   
                   
               
             
          
         
       
     
     Generally, the division between indirect and direct semiconductor materials is at about 41-43%. Higher than that is indirect, lower is direct. The exact percentage AL number may vary based on temperature and fabrication. The exemplary embodiment was fabricated at substantially 50% AL and substantially 30% AL. In an alternative embodiment fabricated using AlGaAs, a percentage greater than 50% AL and lower than 33% AL may be employed. 
     The semiconductor embodiments  200 ,  500 ,  600  may be readily fabricated using any suitable semiconductor fabrication process. Further, any suitable semiconductor material may be used in fabrication of a direct thermal electric converter  100 . Other non-limiting examples of semiconductor materials include, but are not limited to, Ge, Hg 1-x Cd x Te, SiGe superlattice, In x Ga 1-x Sb, GaSb, PbS, PbSe, or PbTe. Indirect narrow-gap superlattice materials, including In x Ga 1-x Sb/Bi y Sb 1-y  may be used. 
     Even when two semiconductors are both direct (or indirect), they may still have different recombination rates. Of particular interest are narrow gap direct semiconductors with different effective Densities of States, indicating different recombination rates. The differing density of state values may be selectively used in the fabrication of the semiconductor layers to control output current and/or voltage. In particular, Lead Sulfide (PbS) and Lead Telluride (PbTe) have a significant differences (for example, a factor of 1.6) at ambient temperature. In general, any pair of materials with a narrow bandgap and unequal recombination rates which can be grown together in the structure may be used in a semiconductor embodiment  200 ,  500 ,  600 . 
     Additionally, or alternatively, the thickness of the semiconductor layers and/or doping layers may be varied to control output current and/or voltage. In some embodiments, additional layers may be inserted between the materials, such as metal layers within the doped regions, without adversely affecting performance. In an exemplary embodiment, the heterojunction is centered between, or is substantially centered between, the doping layers  208   a/b . In other embodiments, the heterojunction may not be centered. In some embodiments, the heterojunction may be located outside the doped layer  208   a/b . In such embodiments, the ratio of carriers may remain dominated by the effects of doping. Also, in such embodiments, one of the doped layers  208   a/b  would be inherently omitted. 
       FIG. 6  is a block diagram of an alternative semiconductor embodiment  600  of the direct thermal electric converter  100 . The semiconductor embodiment  600  comprises a plurality of alternating layers of a high recombination material  204  and a low recombination material  202  which are separated by either a positive doped layer  206   a/b  or a negative doped layer  208   a/b . A positive terminal  210  and a negative terminal  212  provide attachment points for delivery of the generated DC electrical power. In alternative embodiments, a positive doped layer  214  and/or a negative doped layer  216  may be optionally included at the ends of the semiconductor embodiment  200  depending upon the material recombination type of the ending portions of the semiconductor materials. 
       FIG. 7  is a block diagram of an electrochemical embodiment  700  of the direct thermal electric converter  100 . The electrochemical embodiment  700  comprises an enclosure  702  enclosing a plurality of alternating layers of a high recombination material  704  and a low recombination material. The low recombination material is cooperatively formed by an anion membrane  706  and a cation membrane  708  in contact with each other. A positive terminal  210  and a negative terminal  212  provide attachment points for delivery of the generated DC electrical power. 
     In an exemplary embodiment, the high recombination material  704  is water. Preferably, the water of the high recombination material  704  is pure, or substantially pure. In some embodiments, chemical additives may be added to adjust the recombination. Alternatively, or additionally, another type of high recombination fluid or material may be used for the high recombination material  704 . 
     When heat energy is added to the electrochemical embodiment  700 , positive charge carriers and negative charge carriers are generated in the low recombination material cooperatively formed by the anion membrane  706  and the cation membrane  708 . The negative charge carriers migrating into the water from the cation membrane  708  recombine with the positively charged carriers migrating into the water from the anion membrane  706  located on the opposing side of the water. 
     In an exemplary electrochemical embodiment  700 , the positive charge carriers are hydrogen ions (H+). The hydrogen ions migrate towards their respective high recombination material  704 , the water. The negative charge carriers in the electrochemical embodiment  700  are hydroxyl ions (OH−). The hydroxyl ions also migrate towards their respective high recombination material  704 , the water. Movement of these positively charged hydrogen ions, and the opposite movement of the hydroxyl ions, results in a net migration of charge across the electrochemical embodiment  700 , thereby resulting in a current and a voltage. The hydrogen ions are opposite in polarity from the hydroxyl ions. 
     Any suitable anion exchange membrane material may be used for the anion membrane  706 . Any suitable cation exchange membrane material may be used for the cation membrane  708 . An exemplary working embodiment employed an AMI-7001S Anion exchange membrane sandwiched with a CMI-7002 Cation Exchange membrane. Nine membrane pairs were arranged in a tray  702  and distilled water was used as the high recombination material  704   
     In the exemplary embodiment, hydrogen was exhausted from the enclosure  702  in proximity to the positive terminal  210 . In some embodiments, the enclosure  702  may be configured to capture the exhausting hydrogen for use in other chemical or electrochemical processes. 
     In the exemplary embodiment, oxygen was exhausted from the enclosure  702  in proximity to the negative terminal  212 . In some embodiments, the enclosure  702  may be configured to capture the exhausting oxygen for use in other chemical or electrochemical processes. 
     In an electrochemical embodiment  700  utilizing water as the high recombination material  704 , the exhausting hydrogen and oxygen deplete the water. Accordingly, water may have to be added from time to time to extend the useful life of the electrochemical embodiment  700 . 
     It should be emphasized that the above-described embodiments of the direct thermal electric converter  100  are merely possible examples of implementations of the invention. Many variations and modifications may be made to the above-described embodiments. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.