ES2690162B2 - THERMOPOTOVOLTAIC CONVERTER - Google Patents

Description

DESCRIPTION

THERMOPOTOVOLTAIC CONVERTER

TECHNICAL SECTOR

The invention belongs to the sector of the direct transformation of heat into electricity through photovoltaic devices.

BACKGROUND OF THE INVENTION

Thermo-photovoltaic converters are devices that are used for the conversion of heat from very high temperature (above 1000 ° C) into electricity. The underlying mechanism consists in the direct conversion of the thermal radiation (photons) into electricity through the photovoltaic effect. For this, semiconductors are typically used that are capable of absorbing low-energy photons and converting them into electrons that move freely within the semiconductor. Examples of these materials are gallium antimonide, gallium-indium arsenide or germanium. The fundamental elements of a thermophotovoltaic converter are two: the thermal emitter and the photovoltaic cell. The geometric disposition of one element with respect to the other is key when it comes to adapting to the specific application in which we are going to use the converter.

In a possible configuration of thermo-photovoltaic converter (TPV Tube Generators for Apartment Building and Industrial Furnace Applications, " AIP Conf. Proc., Vol 653, no.1, p.

38-48, Jan. 2003 [1], US4419532 A, US20150256119 A1), the thermal emitter are the internal walls of a cavity that is heated through its external surface by a heat source (combustion, solar energy, etc.). ). The internal walls of said cavity, or emitter, are visible by at least one photovoltaic cell, placed inside said cavity. The emitter emits thermal radiation that is captured by the photovoltaic cell, which consequently produces electricity. This configuration allows capturing the heat of the environment in which the converter is located, which is why it is indicated for its application in the heat recovery in industrial furnaces (as described in [1]) or in thermal energy storage systems. (as described in US4419532 A or in US20150256119 A1).

The designs described above are designed to use photovoltaic cells with two electrical contacts, one located on the front surface and another located on the back surface of said cell. The frontal contact is partially transparent to allow the absorption of thermal radiation, while the back contact is completely opaque and is in direct contact with a substrate, whose main purpose is to confer mechanical stability and facilitate the dissipation of the heat generated in the cell . Said substrate is in contact with a heat sink that can incorporate ducts through which it flows in refrigerant fluid, so that it maintains the photovoltaic cell at temperatures no higher than 100 ° C.

As in these systems the thermal radiation coming from the emitter reaches the cell with 360 ° viewing angle, it is necessary to have at least two cells, each capable of capturing a viewing angle of 180 ° by its frontal surface. These cells must be located on the opposite sides of the heat sink, so that the receiver formed by both cells is capable of capturing radiation from a viewing angle of 360 °.

The main problem with this configuration is the high cost of using two photovoltaic cells to capture all the radiation coming from the emitter. Therefore, the object of this invention is to provide a thermo-photovoltaic converter that requires a single cell capable of capturing the radiation coming from a viewing angle of 360 °, and which at the same time has a heat dissipation system that allow to maintain the cell at temperatures not higher than 100 ° C.

DESCRIPTION OF THE INVENTION

The present invention provides a thermophotovoltaic converter for the direct conversion of heat into electricity in a configuration in which the emitter is the internal walls of a cavity that completely surrounds the photovoltaic cell, and in which a single photovoltaic cell is capable of capturing radiation from the entire surface of the emitter, and in which a method of heat dissipation is available that allows to cool the photovoltaic cell.

This device allows halving the surface of semiconductor material to generate the same electrical power and, at the same time, increase the efficiency of heat conversion in electricity. The final result is a lower cost of the electricity generated.

For this, the thermophotovoltaic converter of the invention comprises at least one photovoltaic cell (5) consisting of two faces arranged to absorb radiation; a conduit (7) for the circulation of a cooling fluid with transparent walls (6), where the conduit (7) surrounds the photovoltaic cell and the walls (6) are at least partially in contact with both faces of the photovoltaic cell ( 5); and a thermal emitter (1), preferably cylindrical, which surrounds the above elements. In the converter of the present invention, the walls (6) are transparent to the radiation emitted by the emitter (I), thus allowing the passage of photons through it without being absorbed.

In preferred embodiments of the present invention, the photovoltaic cell (5) is formed by a photo-generating sheet (2) that generates electron-hollow pairs from the absorption of the photons emitted by the emitter (1). Said sheet consists of two faces, in which two materials have been deposited, partially transparent in the spectral absorption range of the photo-generator sheet (2), which perform the function of selective contacts for electrons (3.1) and holes (3.2). ), respectively. An additional layer of electrically conductive material, also semi-transparent (4), is deposited on these layers, whose function is to transport the electrons outgoing from the selective contact of electrons (3.1) and the incoming electrons to the selective contact of holes (3.2). The photovoltaic cell (5) formed by the elements (2), (3.1), (3.2) and (4) is in contact with at least part of the walls (6) of a conduit (7) for the circulation of a fluid refrigerant, where these walls (6) are transparent to the radiation emitted by the emitter (1), at least in the spectral range corresponding to the spectral response of the photo-generator sheet (2), allowing the passage of photons through it without being absorbed.

The emitter (1) is heated directly by an external thermal source (sunlight, combustion, nuclear reaction, etc.) and emits photons on its inner side. The emitted photons (10) that have enough energy to be absorbed in the photo-generator sheet (2) generate an electron / hollow pair inside it. The electrons are collected by the selective contact of electrons (3.1) and the holes by the selective contact of holes (3.2). The holes that arrive at the selective contact of holes (3.2) recombine with the incoming electrons to said contact through the conductive layer (4), allowing the flow of current through the terminals positive (8) and negative (9) of the converter. The photons (II) that do not have enough energy to be absorbed in the photo-generator sheet (2) can be reabsorbed in the opposite wall of the emitter (1). In this way, said photons they can be recycled and help keep the emitter warm (1). However, part of the photons emitted by the emitter (1) can be absorbed in one of the intermediate elements (2), (3.1), (3.2), (4) or (6), contributing to increase the temperature of said elements, and with them the photo-generator sheet (2). To prevent said sheet from reaching excessively high temperatures, preferably not higher than above 100 ° C, since these temperatures would end up damaging the device and reducing the conversion efficiency, a cooling fluid is circulated through the conduit (7), which It is responsible for transferring the heat generated by the absorption of these photons to the outside. This fluid is transparent to the radiation emitted by the emitter (1) at least in the spectral range corresponding to the spectral response of the photo-generating sheet (2), allowing the passage of photons through it without being absorbed.

This invention can be used in two main types of applications: energy recovery systems in industrial processes and energy storage systems at very high temperatures, preferably higher than 1000 ° C, for example, in concentrating thermosolar generation systems. It can also have application in the generation of electricity in space applications, especially in those missions that are carried out close to the Sun and where the solar radiation is too intense to use conventional solar cells. In all of them, this invention would reduce the cost and improve the functionality of these power generation systems.

BRIEF DESCRIPTION OF THE FIGURES

In order to help a better understanding of the characteristics of the invention and to complement this description, the following figures, whose character is illustrative and not limiting, are accompanied as an integral part thereof:

Fig. 1 shows a cross section of a thermophotovoltaic converter according to the invention.

Fig. 2 shows a cross section of a thermophotovoltaic converter according to another embodiment in which the converter consists of three photovoltaic cells connected in series, so that the positive terminal of one of the cells is connected to the negative terminal of the next, for which the position of the selective contacts for electrons (3.1) and holes (3.2) is alternated in the adjacent cells of the converter.

Fig. 3 shows a top view of the thermophotovoltaic converter.

DETAILED DESCRIPTION OF THE INVENTION

The emitter (1) comprises a refractory material. In a possible realization, the emitter can be manufactured by refractory metals of high melting point (higher than 1000 ° C) and a relatively low vapor pressure (<10-9 atm at 1000 ° C), such as tungsten, molybdenum, tantalum or platinum. In another possible embodiment, the emitter (1) is manufactured using a refractory ceramic material such as silicon carbide, graphite or alumina.

In a possible embodiment, the surface of the internal cavity of the emitter (1) is covered with a layer of a refractory material that gives the surface properties of selective thermal emission, so that most of the photons emitted by the emitter have energies high enough to produce the generation of an electron-hole pair in the photo-generator sheet (2). This layer may consist of a Yterbio (Yb2O3) oxide or Erbium oxide (Er2O3) of between 1 and 10 microns in thickness.

The photo-generator sheet (2) comprises a semiconductor material whose banned bandwidth is narrow enough to allow absorption of the photons emitted by the emitter in the temperature range between 1000 and 2000 ° C. In a possible embodiment, this sheet can be manufactured by crystalline semiconductors that consist of a single chemical element, such as silicon or germanium, whose absorption thresholds are 1.12 eV and 0.66 eV, respectively. In another possible embodiment, this sheet can be manufactured with binary, ternary or quaternary crystalline semiconductor materials such as GaSb, InGaAs or InGaAsSb, whose absorption thresholds are 0.73 eV, 0.74 eV and 0.5 eV, respectively.

The selective contacts for electrons (3.1) and holes (3.2) comprise a semiconductor material. In a particular embodiment of the invention, the selective contacts can be manufactured from the same semiconductor that has been used to manufacture the photo-generating sheet, but incorporating N-type dopants to manufacture the selective contact of electrons (3.1) or P-type dopants to manufacture the selective contact of holes (3.2). If the photo-generator sheet has been manufactured in silicon or germanium, the N-type dopant may be Phosphorus and the P-type dopant may be Boron. In another particular embodiment, the contacts Selective materials can be manufactured with a semiconductor material with a banned bandwidth greater than that of the semiconductor used to manufacture the photo-generator sheet (2) and with the necessary doping (type N or P) to function as selective electron contact (type N) or holes (type P). In the case of a photo-generator plate of silicon or germanium, the selective contacts can be manufactured with amorphous silicon doped with phosphorus to make the selective contact of electrons (3.1), or doped with boron for the selective contact of holes (3.2).

The conductive layer (4) can have a variable thickness from 100 to 2000 nm, and be manufactured by a transparent conductive oxide, such as zinc oxide doped with aluminum (ZnO: Al) or the indium oxide doped with tin (ITO) ). In another particular embodiment, the conductive layer 4 can comprise a metal mesh placed on the transparent conductive oxide, in order to improve the electrical conduction properties of said layer. Likewise, it is possible to dispense with the transparent conductive oxide and to use only the metallic mesh, directly placed on the selective contacts of electrons (3.1) or holes (3.2). In any of the above embodiments, said metal mesh can be manufactured by metals of high electrical conductivity such as silver, aluminum, copper or gold.

The walls (6) of the conduit (7) can be manufactured in quartz or glass, and the union with the photovoltaic cell (5) can be realized by means of a transparent optical epoxy, so that a good thermal and optical contact is made between both faces of the photovoltaic cell (5) and the walls (6) of said conduit (7).

The cooling fluid that is introduced through the conduit (7) can be a liquid with an ebullition point comprised between 50 ° C and 400 ° C, and is at least partially transparent to the radiation in the spectral range corresponding to the absorption in the photogenerator sheet (2). In a particular embodiment of the invention, distilled water is used as a cooling fluid, since it is transparent to radiation with wavelengths between 200 and 1000 nm, and its boiling point at atmospheric pressure is 100 ° C. The use of water as a cooling liquid could be combined with a photogenerator sheet (2) of silicon, whose spectral response ranges from 400 to 1100 nm. In another particular embodiment, the fluid may consist of liquids of higher boiling point such as glycerol (290 ° C), ethylene glycol (197 ° C) or therminol (359 ° C). These fluids, together with other lower melting point fluids such as methanol (65 ° C), ethanol (78 ° C), Isopropyl alcohol (83 ° C), or ethyl ethanoate (77 ° C), have the advantage of having a greater transmittance in the window from 800 to 1400 nm, where the spectral response of a silicon photo-generating sheet is greater .

In particular embodiments of the present invention, the thermophotovoltaic converter described in this patent application comprises two or more photovoltaic cells (5) connected in series. In particular, the photovoltaic cells are connected in such a way that the positive terminal (8) of one of the cells is connected to the negative terminal (9) of the next one, for which the position of the electron-selective contacts is alternated (3.1 ) and gaps (3.2) in the adjacent cells of the converter. In particular, Figure 2 shows the converter of the invention with three photovoltaic cells (5) in series.

The embodiments that include two or more cells connected in series allow to increase the output voltage of the converter and at the same time increase the area of energy capture of the converter. In this way, it is possible to increase the size of the converter and thus the power generated, without the need to increase the output current, which would lead to very high ohmic losses.

Claims (15)

1. - A thermophotovoltaic converter characterized because it comprises: - at least one photovoltaic cell (5) consisting of two faces arranged to absorb radiation; - a conduit (7) for the circulation of a cooling fluid with transparent walls (6), which surrounds the photovoltaic cell, where the walls (6) are at least partially in contact with both faces of the photovoltaic cell (5); Y - a thermal emitter (1) that surrounds the previous elements. 2. - The thermophotovoltaic converter of claim 1, wherein the photovoltaic cell (5) comprises: • a photo-generator sheet (2) of electron-hollow pairs consisting of two faces; • a layer of partially transparent material in the spectral absorption range of the sheet (2), with a selective contact function for electrons (3.1), deposited on one of the faces of the photo-generating sheet (2); • a layer of partially transparent material in the spectral absorption range of the sheet (2), with function of selective contact for holes (3.2), deposited on the other side of the photo-generating sheet (2), and • layers of semi-transparent electrical conductive material (4) deposited on layers (3.1) and (3.2). 3. The thermophotovoltaic converter according to claim 2, wherein the photogenerator sheet (2) comprises a crystalline semiconductor material selected from the group consisting of silicon, germanium, GaSb, InGaAs and InGaAsSb. 4. The thermophotovoltaic converter according to any one of claims 2 to 3, wherein the layer of material with selective electron contact function (3.1) comprises the same semiconductor material as the photo-generator sheet (2) together with N-type dopants; Y the layer of material with function of selective contact for holes (3.2) comprises the same semiconductor material as the photo-generator sheet (2) together with dopants type P. 5. The thermophotovoltaic converter according to claim 4, wherein the semiconductor material is selected from silicon and germanium, the N-type dopant from layer (3.1) is phosphorus and the P-type dopant from layer (3.2) is Boron. 6. - The thermophotovoltaic converter according to any one of claims 2 to 3, wherein the material layer with electron selective contact function (3.1) comprises a semiconductor material with a prohibited bandwidth greater than that of the semiconductor comprised in the photo-generator sheet (2) and type N dopants; Y the layer of material with function of selective contact for holes (3.2) comprises a semiconductor material with a prohibited bandwidth greater than that of the semiconductor comprised in the photo-generator sheet (2) and dopants type P. 7. - The thermophotovoltaic converter according to claim 6, wherein the semiconductor material of the photo-generating sheet (2) is selected from silicon and germanium, the semiconductor material of the layers (3.1) and (3.2) is amorphous silicon, the dopant Type N of the layer (3.1) is Phosphorus and the dopant type P of the layer (3.2) is Boron. 8. The thermophotovoltaic converter according to any one of claims 2 to 7, wherein the conductive layer (4) comprises a transparent conductive oxide, a metal mesh or a combination of both. 9. - The thermophotovoltaic converter of claim 8, wherein the metal mesh is formed with a metal selected from the group consisting of silver, aluminum, copper and gold. 10. - The thermophotovoltaic converter according to any one of claims 1 to 9, wherein a layer of refractory material with selective thermal emission properties is deposited on the surface of the internal cavity of the emitter (1). 11. The thermophotovoltaic converter according to claim 10, wherein the refractory material is selected from ysterbio oxide and erbium oxide, and the layer has a thickness between 1 and 10 microns. 12. - The thermophotovoltaic converter according to any one of claims 1 to 11, wherein the walls (6) are at least partially in contact with both sides of the photovoltaic cell (5) by a transparent optical epoxy. 13. The thermophotovoltaic converter according to any one of claims 2 to 12, wherein the cooling fluid circulating through the conduit (7) is a liquid boiling point between 50 ° C and 400 ° C, partially transparent to the radiation in the spectral range of absorption of the photo-generator sheet (2). 14. The thermophotovoltaic converter according to any one of claims 1 to 13, comprising two or more photovoltaic cells (5) connected in series. 15. Use of the thermophotovoltaic converter that is described in any of claims 1 to 14 in energy recovery systems in industrial processes, energy storage systems at temperatures above 1000 ° C or in electricity generation systems in space applications.