Patent Publication Number: US-2003230336-A1

Title: Thermophotovoltaic conversion module and apparatus thereof

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates generally to a thermophotovoltaic (TPV) conversion module and, more particularly, to an improved TPV conversion module, which is suitable for the use in systems for the distributed generation of electric power.  
       BACKGROUND OF THE INVENTION  
       [0002] It is known that systems for the distributed generation of electric power have had a large diffusion in the market, in the last years. Distributed generation (DG) systems are mainly aimed at generating electric power in the proximity of the final user, avoiding the need of an electric power distribution network. Distributed generation of electric power is generally adopted also in combined-heat-power (CHP) systems and in combined-heat-cold-power (CHCP) systems, which refer to the simultaneous production of electric power, hot water and steam for heating or cooling purposes.  
       [0003] A wide range of electric power may be covered: from few hundreds watts (domestic applications) to several megawatts (huge industrial plants), depending on the needs.  
       [0004] In the past, there have been, in the field of the distributed generation, different types of electric power generation methods, which employ generally internal combustion engines (ICE) or turbo-machines.  
       [0005] These traditional methods have some disadvantages, which are mainly related to the fact of using apparatuses, provided with moving parts and, therefore, subjected to wear problems, high noise levels, frequent maintenance interventions and the like.  
       [0006] Beside these traditional methods, some alternative methods, which imply a “static” generation of electric power, have been developed. Among these, the methods, which adopt solar photovoltaic (PV) conversion for generating electric power, have to be mentioned. Solar photovoltaic converters are extensively used in low-medium scale electric power distributed generation systems. Thanks to absence of moving parts, they allow overcoming most of the disadvantages that are proper of the methods employing ICE or turbo-machines.  
       [0007] Similarly to the PV conversion, also to the TPV electric power conversion prevents from the use of moving parts within an electric power generator. In fact, TPV electric power generation is based upon the positioning semiconductor TPV cells, which are structured similarly to the well known solar cells, in the photon flux field of a high temperature photon emitter. In practice, TPV electric power generation is based on the conversion of infrared (IR) radiation, which is emitted by a heat source. The development of emitters, able to emit a narrow-band IR radiation, and the adoption of TPV cells, made of selected semiconductor compounds, such as those of the III-V families (GaAs, GaSb, InGaSb, InGaAsSb and the like), have allowed reaching high power density levels (higher than 1W/cm 2 ). Therefore, TPV electric power generation has shown to be attractive for the relatively low size and the relatively reduced installation and maintenance costs of the TPV generation systems, keeping unchanged, at the same time, the advantages that are proper of the PV technology.  
       [0008] Traditionally, TPV generation systems adopt a configuration, which is similar to that one illustrated in FIG. 1. A TPV apparatus comprises a TPV conversion module  100  that includes an emitter  1 , which is provided generally with a cylindrical structure, similar to the radiant tubes that are traditionally used in industrial kiln-drying systems. The emitter  1  is associated to a burner (not illustrated) which generally constitutes the heat source and is supplied with fuel  8  and combustion air  7 . The combustion process obviously provokes the emission of hot gases from the burner. These hot gases are provided with a certain amount thermal power, which increases the temperature of the emitter  1 . Thus, the emitter  1  emits externally IR photons  3 . Further, a TPV conversion module comprises also a receiver  200 , which comprises semiconductor TPV cells arrays  201  for converting the IR radiation  3  into electric power  4 . A recuperator  5  is generally associated to the burner. The recuperator  5  recovers the residual heat of the flue gases  6  in order to increase the whole efficiency of the conversion system. In fact, by means of heat exchangers, it is possible to pre-heat the air, which is used for the combustion (the pre-heated air flux is represented by arrow  9 ).  
       [0009] Unfortunately, traditional TPV systems are affected by some drawbacks. A first drawback is due the relatively high production costs of the TPV conversion modules that are generally adopted. As mentioned, a traditional TPV conversion module comprises TPV cells that are made of inorganic semiconductor compounds. TPV cells are manufactured adopting technologies that derive from the semiconductor industry. A traditional manufacturing technique is substantially based on the epitaxial growth of semiconductor layers (generally made of AlGaAs or InGaAs or InGaAsSb) on a semiconductor substrate (generally made of GaAs, InP or GaSb). This manufacturing technique is quite sophisticated and requires expensive apparatuses, skilled personnel and expensive time-consuming manufacturing procedures. Thus, in practice, this manufacturing technique is use for high value and high cost applications, only. An alternative traditional manufacturing technique is based on the diffusion of dopants on a suitable semiconductor substrate (generally made of GaSb). This manufacturing technique requires cheaper arrangements and, for this reason, is widely adopted for production of TPV cells. However, manufacturing costs are still relatively high and massive production volumes are needed for achieving reasonable cost targets.  
       [0010] Another important drawback derives from the fact that the use of inorganic semiconductor cells intrinsically implies severe constraints to the design of a TPV module. For example, due to the fact that TPV cells have small size due to manufacturing constraints (e.g. intensive utilization of the wafer available surface), a large number of TPV cells have to be serially connected in order to achieve satisfactory levels of output electric power. This requires setting up complicated circuit arrangements. Further, the overall weight of the TPV conversion module is often quite remarkable and, therefore, the assembly of a TPV apparatus, which comprises one or more TPV conversion modules, may be quite difficult. Moreover, inorganic TPV cells are intrinsically rigid and, therefore, it is very difficult to dispose them of curved surfaces. In a TPV conversion module, this may provoke a decrease of the power density with reference to the power transmitted from the emitter body to the receiver body and, consequently, a decrease of the overall conversion efficiency.  
       [0011] Finally, the realization of inorganic TPV cells provided with multi-layered structures, which would improve the conversion efficiency of the TPV conversion module, is quite difficult and expensive, due to the fact that complicated manufacturing technologies have to be adopted.  
       [0012] In practice, one can state that the use of inorganic TPV cells, in traditional TPV conversion modules, does not allow achieving fully satisfactory performances in terms of manufacturing costs, design flexibility and performances, yet.  
       SUMMARY OF THE INVENTION  
       [0013] The main aim of the present invention is to provide a TPV conversion module, which allows overcoming the mentioned drawbacks. Within this aim, an aspect of the present invention is to provide a TPV conversion module, which can be manufactured with relatively low-cost processes. Another aspect of the present invention is to provide a TPV conversion module, which can be designed with a high level of flexibility and reliability. Another aspect of the present invention is to provide a TPV conversion module, which allows achieving improved performances, particularly in terms of conversion efficiency.  
       [0014] Thus, the present invention provides a TPV conversion module, which comprises an emitter body, which is associated to a burner device that generates hot gases provided with an amount of thermal power. The emitter body comprises at least an emitting surface, which emits IR radiation, when the emitter body is heated by said hot gases. Further, the TPV conversion module, according to the present invention, further comprises a receiver body, which is associated to the emitter body. The receiver body comprises a receiving surface, which receives, at least partially, the IR radiation. One or more TPV cells are disposed on this receiving surface for converting, at least partially, the IR radiation into electric power.  
       [0015] The TPV conversion module, according to the present invention, is characterized in that TPV cells comprise at least a TPV cell, which include a multi-layered structure. This multi-layered structure comprises at least a first layer of organic material.  
       [0016] For a better understanding of the present invention, reference is made to the accompanying drawings and to the detailed description reported hereinafter, in which preferred embodiments of the invention are illustrated. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0017] In the drawings:  
     [0018]FIG. 1 represents schematically the structure of a known TPV apparatus;  
     [0019]FIG. 2 represents schematically the basic structure of the TPV conversion module, according to the present invention;  
     [0020]FIG. 3 represents schematically a preferred embodiment of the TPV conversion module, according to the present invention;  
     [0021]FIG. 4 represents schematically another preferred embodiment of the TPV conversion module, according to the present invention;  
     [0022]FIG. 5 represents schematically the basic structure of a TPV apparatus, which incorporates a TPV conversion module, according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0023] Referring to FIG. 2, the TPV conversion module  16 , according to the present invention, generates electric power and comprises an emitter body  100 , which is associated to a burner device (not shown). The burner device generates hot gases  13  that are provided with an amount of thermal power. The emitter body  100  comprises at least an emitting surface  501 , which emits IR radiation  101 , when the emitter body  100  is heated by the hot gases  13 . The TPV conversion module  16  comprises also a receiver body  102 , which is associated to the emitter body  100 . The receiver body  102  comprises a receiving surface  502 , which receives, at least partially, the IR radiation  101 . One or more TPV cells  103  (dotted line) are disposed on the receiving surface  502  for converting, at least partially, the IR radiation  101  into electric power. The peculiarity of the TPV conversion module, according to the present invention, consists of the fact that the TPV cells  103  comprises at least a TPV cell, which includes a multi-layered structure  600 . The multi-layered structure  600  comprises at least a first layer  601 , made of organic material. The organic material may belongs preferably to the PPV (polyphenylenevinylene) and/or PAn (Polyaniline) and/or PA (Polyacetylene) and/or PPy (polypirrole) and/or PTh (Polythiophene) and/or P3MTh (polymethilthiophene) and/or PVCZ (Ployvinylcarbonazole) families. Preferably, the first layer  601  is doped, in order to provide the organic material with a band gap lower than 1 eV, suitable to receive the IR radiation  101 . Doping materials may be atoms/molecules chemically bonded to the polymer base or fine particles physically bonded to the polymer matrix (e.g. nanostructured particles such as fullerenes). In any case, the object of the doping is to create a suitable energy gap (lower than 1 eV) between the π-ligand and π*-antiligand orbitals in the organic material of the first layer  601 . In this manner, an amount of free charge carriers is generated in the first layer  601 , when it receives at least a portion of IR radiation  101 .  
     [0024] Preferably the multi-layered structure comprises a first electrode  602 , associated to the layer  601 . Further, the multi-layered structure  600  comprises a second layer  603  associated to the first layer  601  and a second electrode  604 , associated to the second layer  603 . In a preferred embodiment of the TPV conversion module (reference  600   a ), according to the present invention, the second layer  603  is made of organic material (preferably belonging to the PPV family) and is associated to the first layer  601 , so as to constitute an p-n heterojunction. As it is known, with the term “heterojunction” it is meant a junction between two materials that may have different structure. At the interface between layers  601  and  603  (i.e. the p- and n-layers of the heterojunction), free charge carriers (holes and electrons) are generated. Then, electrodes  602  and  604  can collect these free charge carriers. Alternatively (reference  600   b ) the multi-layered structure  600  may comprise a third layer  605 , which is interposed between said first layer  601  and the first electrode  602 . The third layer  605  may comprise advantageously an electrolitic solution (which might be in a liquid, solid or gel state) for providing an exchange of free charge carriers (mainly electrons) between the first layer  601  and the first electrode  602 . In fact, ions of the electrolitic solution are able to chemically reduce the molecules of the layers  601  and, therefore, to generate free electrons that are driven towards the electrode  602 . Advantageously, a fourth layer  609  may be interposed between the first layer  601  and the third layer  605 . The fourth layer  609  (preferably made of Pt) works as a catalyst, aimed at improving the exchange of free charge carriers (mainly electrons) between the first layer  601  and the first electrode  602 . Preferably, also a fifth layer  606  may be interposed between the second layer  603  and the second electrode  604 . The fifth layer  606  (which preferably is made of TiO 2 ) is aimed at improving the exchange of free charge carriers (mainly electrons) between the second layer  603  and the second electrode  604 .  
     [0025] From what described above, it appears evident that the multi-layered structure  600  is able to converts the received IR radiation into electric power and, therefore, it is able to substitute the traditional inorganic TPV cells that are commonly adopted. Further, due to the adoption of a plurality of active layers, it is able to provide a remarkably improved conversion efficiency of the TPV cell. In a preferred embodiment to the present invention (not illustrated), the TPV conversion module  16  comprises a cooling system, which is associated to the receiver body  102 . The cooling system is aimed at maintaining the operating temperature of the plurality of TPV cells  103  within a predefined temperature range. This is quite advantageous, because the TPV cells are allowed to work always at optimal operating conditions.  
     [0026] The use of organic TPV cells  103 , and, in particular, of the multi-layered structure  600  allows adopting various advantageous configurations for the TPV conversion module  16 .  
     [0027] According to a preferred embodiment, illustrated in FIG. 2, the emitter body  100  comprises at least a first flat wall  401 , which includes the emitting surface  501 . Accordingly, the receiver body  102  comprises at least a second flat wall  402 , which includes the receiving surface  502 .  
     [0028] According to another preferred embodiment (FIG. 3), the emitter body  100  comprises at least a first through cavity  21 , in which the hot gases  13  are allowed to flow. The emitter body  100  is therefore heated because of the flow of the hot gases  13 . The emitter body  100  comprises also at least an external wall  22 , which advantageously includes the emitting surface  501 . According to the same preferred embodiment, the receiver body  102  comprises a second through cavity  25 , in which the emitter body  100  is positioned. The internal wall  26  of the second through cavity  25  advantageously includes the receiving surface  502  and, therefore, it supports a plurality of TPV cells (hatched box  103 ) that are positioned so as to receive the IR radiation  101 . Advantageously, the second through cavity  25  can be sealed so as to obtain a vacuum atmosphere inside the cavity or an atmosphere filled with a noble gas (e.g. Helium).  
     [0029] The emitter body  100  can advantageously comprise layers of high emissivity material such as, for example, SiC or Kanthal.  
     [0030] Preferably, the receiver body  102  comprises at least an external wall  28 , which exchanges heat with a cooling fluid (not illustrated). The cooling fluid is advantageously supplied by the mentioned cooling system.  
     [0031] Referring to FIG. 4, according to another preferred embodiment of the present invention, the emitter body  100  is exposed to the flow of hot gases  13 , so as to be heated. The emitter body  100  comprises a third through cavity  31 . At least an internal wall  32  of the third through cavity  31  includes the emitting surface  501 , which emits an IR radiation  101 , when the emitter body  100  is heated by the hot gases  13 . The third through cavity  31  can be sealed so as to obtain a vacuum atmosphere inside the cavity or an atmosphere filled with a noble gas (e.g. Helium).  
     [0032] According to the same preferred embodiment, the receiver body  102  is positioned inside the third through cavity  31 , so as to be surrounded by the emitter body  100 . At least an external wall  35  of the receiver body  102  includes the receiving surface  502 .  
     [0033] The receiver body  102  comprises advantageously a fourth through cavity  37 , through which a cooling fluid (not illustrated) can flow and exchange heat with at least an internal wall  38  of the fourth cavity  37 . The cooling fluid can be supplied by the cooling system  104  of FIG. 3.  
     [0034] These configurations represent a substantial improvement in terms of conversion efficiency. The geometry of the emitter body  100  and the receiver body  102  can be designed, according to the needs. For example, the may have a cylindrical geometry as illustrated in FIGS. 3 and 4 or they may have a polyedric geometry. It should be noticed that, thanks to the intrinsic flexibility of the organic TPV cells, no substantial design constraints derive from the need of disposing the TPV cells  103  on the receiving surface  502 .  
     [0035] Referring now to FIG. 5, a preferred embodiment of a TPV apparatus (reference  10 ), which incorporates the TPV conversion module  16 , according to the present invention, is illustrated. The TPV apparatus  10  comprises preferably a combustion chamber  11 . The combustion chamber  11  comprises a burner device  12 , which generates by means of the combustion of fuel  76  and combustion air  71  hot gases  13 , which provide an amount of thermal power. The TPV apparatus  10  comprises also a converter assembly (globally indicated by the dotted box  14 ), which is associated to the combustion chamber  11 . The converter assembly  14  converts into electric power at least a portion of the amount of thermal power, which is provided by the hot gases  13 . The converter assembly  14  comprises one or more TPV modules  16 . Preferably, the converter assembly  14  (and, therefore, the TPV conversion modules  16 ) may be positioned internally to the combustion chamber  11 . This configuration is quite advantageous, since it allows increasing the output power of the TPV apparatus, which can be quite higher than 1 KW.  
     [0036] The TPV apparatus  10  comprises preferably a recuperator assembly  70 . The recuperator assembly  70  can perform a pre-heating of the combustion air  71 , by means of the residual heat of the hot gases  13 , simply adopting exchanger tubes  73 . Alternative, the burner device  12  of the TPV apparatus  20  may be advantageously provided with an auto-recuperative system (not illustrated), for directly pre-heating the combustion air  71 . Therefore, the structure of the TPV apparatus  10 , according to the present invention, can be remarkably simplified. Alternatively, the burner device  12  can comprise an auto-regenerative burner, such as one of those generally used in industrial kiln-drying systems. In any case, the total efficiency of the TPV apparatus  80 , according to the present invention, can be further increased till to relatively high levels (around or more 20%). Moreover, the burner device  12  may comprise a burner of the flame-type or, more advantageously, of the flame-less type, so as to reduce the NOx emissions of the TPV apparatus  20 .  
     [0037] The TPV apparatus  10  is quite advantageous because, due to its modular structure, whose size basically depends on the number of adopted TPV conversion modules, it allows to provide an output power level, which can be simply modulated according to the needs.  
     [0038] It has been proved in practice that the TPV conversion module, according to the present invention, allows achieving all the intended aims and objects. Thanks to the use of organic TPV cells it is possible to reduce drastically the manufacturing costs of the TPV conversion module, ensuring in the same time high level performances. In fact, the manufacturing technologies of the organic TPV cells are relatively simple to perform and do not require sophisticated arrangements. Multi-layered structures can be fabricated with relatively low efforts. Therefore, the conversion performances of each TPV cells may be optimized.  
     [0039] Further, TPV cells provided with a remarkably surface may be realized: this fact implies that it possible to simplify the structure of the receiver body  102 , which does not require the presence of complicated circuit arrangements, anymore. TPV organic cells, being based on the use of organic materials, are intrinsically provided with a high level of elasticity. Therefore, curved receiving surfaces  502  may be easily realized: this fact allows improving remarkably the overall efficiency of the TPV conversion module and modulating, by means of optimized layouts of the receiving surface  502 , the transmitted power density, according to the needs. Finally, TPV organic cells are intrinsically characterized by a lower weight: this allows improving the assembly of a TPV conversion module  16  and more in general the assembly of TPV apparatus  10 .  
     [0040] Finally, the practice has shown that the TPV conversion module, according to the present invention, allows designing TPV apparatuses that are able to provide remarkably improved performances both in terms of reliability and flexibility of use.