WO2017134321A1 - Hybrid thermionic-photovoltaic converter - Google Patents

Hybrid thermionic-photovoltaic converter Download PDF

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Publication number
WO2017134321A1
WO2017134321A1 PCT/ES2017/070035 ES2017070035W WO2017134321A1 WO 2017134321 A1 WO2017134321 A1 WO 2017134321A1 ES 2017070035 W ES2017070035 W ES 2017070035W WO 2017134321 A1 WO2017134321 A1 WO 2017134321A1
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Prior art keywords
collector
emitter
cell
photons
photovoltaic cell
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PCT/ES2017/070035
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Spanish (es)
French (fr)
Inventor
Alejandro DATAS MEDINA
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Universidad Politécnica de Madrid
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Publication of WO2017134321A1 publication Critical patent/WO2017134321A1/en

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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02S—GENERATION OF ELECTRIC POWER BY CONVERSION OF INFRARED RADIATION, VISIBLE LIGHT OR ULTRAVIOLET LIGHT, e.g. USING PHOTOVOLTAIC [PV] MODULES
    • H02S10/00—PV power plants; Combinations of PV energy systems with other systems for the generation of electric power
    • H02S10/30—Thermophotovoltaic systems
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J45/00—Discharge tubes functioning as thermionic generators
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy

Definitions

  • the invention belongs to the sector of the direct transformation of high temperature heat into electricity by means of solid state devices. BACKGROUND OF THE INVENTION
  • thermo-photovoltaic for the conversion of heat from very high temperature (above 1000 e C) into electricity
  • thermo-photovoltaic two types of devices stand out: thermo-photovoltaic and thermionic.
  • These devices differ in the type of heat carriers they use: electrons (in the case of thermionic converters) or photons (in the case of thermo-photovoltaic).
  • electrons in the case of thermionic converters
  • photons in the case of thermo-photovoltaic
  • the fundamental problem of both devices is that they need to work at extremely high temperatures to provide sufficient electrical power density.
  • variations of these two types of basic converters have been proposed in order to increase the density of electrical power generated by the device. For example, in application US201 10100430 a converter of light radiation into electricity is proposed that combines the photovoltaic and thermionic effects.
  • the converter according to US201 10100430 uses a photovoltaic cell to directly transform the most energetic photons of radiation into electricity and a thermionic converter to transform the less energetic photons, not usable by the photovoltaic cell.
  • the cathode of the thermionic converter is located behind the photovoltaic cell and is heated by absorbing the less energetic photons of the incident radiation, not absorbed by the photovoltaic cell. If the cathode reaches a sufficiently high temperature, it will emit electrons to the anode.
  • the anode is placed on the unlit surface of the photovoltaic cell, that is, between the photovoltaic cell and the cathode, and collects the electrons emitted by the cathode to close the circuit.
  • This device is equivalent to a multi-junction photovoltaic cell with the proviso that low energy photons are indirectly transformed into electricity by a thermionic effect, and not Direct form through a photovoltaic effect. Because it is an indirect conversion (which requires intermediate heat generation at the cathode), the conversion efficiency of the less energetic photons is considerably lower. In addition, the power density attainable by the converter remains limited by the amount of photons emitted by the source of thermal radiation (external element to the device).
  • thermo-photovoltaic device limited by the number of photons
  • thermionic limited by the number of electrons
  • the object of this invention is to achieve a device capable of extracting greater power from the thermal source and at the same time making a direct conversion of heat into electricity in order to increase the density of electrical power generated by the device.
  • the object of the present invention is to provide a thermo-photovoltaic converter for the direct conversion of high temperature heat into electricity, which allows to increase the density of electrical power extracted from the thermal source.
  • the invention comprises an emitter (1), an electron collector (2) and a photovoltaic cell (3).
  • the emitter is heated directly by the thermal source (sunlight, combustion, nuclear reaction, etc.) thanks to the radiation received on a first surface (1 .1). Consequently, the emitter emits photons (4) and electrons (5) simultaneously on a second surface opposite the first (1 .2).
  • the emission of photons depends on the temperature and the emissivity of said surface, while the emission of electrons depends on the temperature and the work function.
  • the collector (2) is responsible for collecting the electrons (5) and producing electric current. Said collector must be transparent, at least partially, to give way to the light radiation emitted by the emitter towards the photovoltaic cell (3).
  • Said cell produces electricity from the photons (4) emitted by the emitter that have not been absorbed by the collector.
  • the advantage of this hybrid system is that it is possible to extract a greater amount of heat from the thermal source and therefore it is possible to increase the density of electrical power.
  • two types of thermal carriers are used (electrons and photons) to transfer the heat output of the source (in intimate contact with the emitter) to the converter.
  • electrons are generated indirectly by absorbing photons from the thermal source. The key is that in this case electrons and photons are emitted directly from the thermal source, and therefore allow to extract much more heat power from it.
  • photons and electrons from the thermal source are directly transformed into electricity by the converter: photons through a photovoltaic effect and electrons through a thermionic effect. Both effects allow to increase the electrical power density and the efficiency of the device.
  • the collector and the cell are two independent elements, but the collector can also be deposited on a substrate and / or on the cell itself.
  • Fig. 1 shows a thermionic hybrid converter- Photovoltaic according to the invention, with independent connection of each element, in which the collector (2) is in an element physically separated from the emitter (1) and the photovoltaic cell (3).
  • the emitter (1) emits photons (4) and electrons (5) to the collector (2).
  • the collector absorbs the electrons and lets the photons into the photovoltaic cell, where they become electricity.
  • the thermionic converter is connected to the outside through terminals (6) and (7), while the photovoltaic cell is connected to the outside through terminals (8) and (9).
  • Fig. 2 shows another possible embodiment in which the collector is deposited directly on the photovoltaic cell, both being electrically connected.
  • Fig. 3 shows an embodiment similar to the previous one, unlike the current is drawn from the device through only two terminals (6) and (9). In this case, the current flowing between the emitter (1) and the collector (2) must be the same as that between the positive (3.1) and negative (3.2) terminals of the photovoltaic cell.
  • the emitter (1) comprises a refractory material.
  • the emitter can be manufactured using high melting metals (above 1700 e C) and a relatively low vapor pressure ( ⁇ 10 ⁇ 9 atm at the working temperature), such as tungsten, molybdenum, tantalum or platinum. It is also possible to use an atmosphere of ionized cesium that occupies the volume between the emitter and the collector, so that the cesium adsorbed on the surface (1 .2) considerably reduces the work function of said surface and facilitates the emission of electrons
  • the emitter (1) is manufactured using a refractory substrate whose sole purpose is to provide mechanical support and transfer heat from the surface (1 .1) to (1 .2).
  • a metallic layer will be deposited on the surface (1 .2) of the emitter to favor the emission of electrons.
  • a cesium atmosphere to reduce the work function of the emitter.
  • the thermal source used to heat the emitter by its surface (1 .1) is light radiation, such as sunlight
  • the collector (2) must have a reduced working function (of the order or less than 1 .5 eV) and less than that of the emitter (to favor the collection of electrons emitted by the emitter) and a high optical transmittance (of the order or greater than 70%, to allow the photons from the emitter to the photovoltaic cell). This last characteristic must be fulfilled at least in a spectral range coinciding with part of the spectral response of the photovoltaic cell (3).
  • a very thin metallic layer (10-100nm) deposited on a substrate (either the cell itself or another support, such as a quartz crystal), which allows the passage of light and at the same time have a metallic behavior that allows the efficient collection of electrons.
  • a sheet in the form of a metallic mesh that allows the passage of light and at the same time allows the collection of electrons, which are selectively directed towards the metallic lines of said mesh.
  • conductive transparent oxides such as indium tin oxide (ITO) or tungsten oxide.
  • ITO indium tin oxide
  • tungsten oxide there are multiple possible configurations for the collector ranging from a simple sheet or metal mesh, to the configuration described above.
  • the photovoltaic cell (3) can be manufactured using at least one semiconductor material (for example Silicon, GaAs, Germanium, GaSb InGaAs, InGaAsSb, etc.) with the optimum bandwidth for the emitter's light emission spectrum (dependent on its temperature) and forming at least one p / n junction (cathode / anode) to make the selective contacts of electrons and holes generated internally in the semiconductor material.
  • semiconductor material for example Silicon, GaAs, Germanium, GaSb InGaAs, InGaAsSb, etc.
  • Said cell will have at least two electrical contacts, one positive (cathode) and the other negative (anode) and may incorporate in its rear surface (3.2) a mirror that returns to the emitter those photons not absorbed by the cell, so as to reduce the amount of heat to dissipate in said cell and in turn the efficiency of the converter is increased.
  • the collector (2) can be an independent element (Fig. 1), be deposited directly on the photovoltaic cell (Fig. 2 and Fig. 3) or be deposited on a transparent substrate (such as glass or quartz). In the latter case, said substrate could, in turn, be placed on the photovoltaic cell (Fig. 2 and Fig. 3) or placed independently, separated from said cell by vacuum or a controlled atmosphere (Fig. 1). In either case, the collector (anode of the thermionic converter) could be electrically connected to the cathode of the photovoltaic cell, both elements being connected in series, so that the current is drawn from the converter between the emitter terminals (6) (cathode of the thermionic converter) and the anode of the photovoltaic cell (9) (Fig. 1)
  • the collector can be placed at a micrometric distance from the surface (1 .2) of the emitter (1) to favor the electron transfer between both elements.
  • the emitter (1) is manufactured in tungsten.
  • the volume between the emitter and the collector is filled with an ionized cesium gas, so that the working function of tungsten is reduced by adsorption of cesium on the surface, reaching a value in the order of 1 .7 eV .
  • an external cesium source will be needed to replace the consumed cesium from the emitter surface.
  • the collector (2) is a thin sheet of tungsten oxide, deposited on a quartz substrate. Tungsten oxide, when adsorbed with cesium in the atmosphere, achieves work functions of the order of 0.75 eV. The thickness of this layer (between 1 and 100nm) is small enough, so that light can pass through it and reach the photovoltaic cell (3).
  • the quartz substrate, which contains the collector, is deposited directly on the photovoltaic cell using a transparent silicone to guarantee the continuity of refractive index between the quartz substrate and the surface of the photovoltaic cell. In this configuration, the face of the quartz substrate containing the collector must face the emitter.
  • the photovoltaic cell (3) is manufactured from a GaSb substrate in which a p / n junction is formed.
  • the GaSb allows photons to be absorbed with energies above 0.7 eV and therefore conforms to the emission spectra corresponding to the emitter's working temperatures (1), between 1000 e C and 1800 e C.
  • the p-type zone (cathode) is located in the front layer of said cell (3.1) to facilitate the eventual connection between the positive terminal of the cell (3.1) and the collector (2) and thus connect in series the thermionic converter with the photovoltaic.
  • the photovoltaic cell On its rear face (3.2) the photovoltaic cell has a reflector that returns the photons not absorbed by the cell to the emitter.
  • This reflector can be manufactured by means of a structure of dielectric layers or by a highly reflective specular metal, such as gold.
  • four electrical contacts can be made (Fig. 1): in the emitter (6), the collector (7), the cathode (8) and the anode of the photovoltaic cell (9), so that the current it is extracted from the device through two independent circuits: one formed by the emitter (6) and collector (7) terminals, and another formed by the cathode (8) and anode (9) terminals of the photovoltaic cell.

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Abstract

The invention relates to a hybrid thermionic-photovoltaic converter for directly converting heat into electricity, said converter comprising: an electron and photon emitter (1) made from a refractory material, an electron collector (2) transparent to photon radiation in the wavelength emitted by the emitter, and a photovoltaic cell (3), said elements being disposed such that the emitter has a first surface (1.1) intended to be oriented towards a heat source and an opposing second surface (1.2) which emits electrons and photons. The collector is positioned facing the second surface such as to receive the electrons emitted by the emitter (1) and the cell (3) is positioned behind the collector (2) such as to receive the photons that pass through the collector.

Description

CONVERTIDOR HÍBRIDO TERMIÓNICO-FOTOVOLTAICO  THERMION-PHOTOVOLTAIC HYBRID CONVERTER
DESCRIPCIÓN SECTOR DE LA TÉCNICA SECTOR DESCRIPTION OF THE TECHNIQUE
La invención pertenece al sector de la transformación directa de calor de alta temperatura en electricidad mediante dispositivos de estado sólido. ANTECEDENTES DE LA INVENCIÓN The invention belongs to the sector of the direct transformation of high temperature heat into electricity by means of solid state devices. BACKGROUND OF THE INVENTION
Entre los convertidores de estado sólido para la conversión del calor de muy a alta temperatura (superior a 1000eC) en electricidad destacan dos tipos de dispositivos: termofotovoltaicos y termiónicos. Estos dispositivos se diferencian en el tipo de portadores de calor que utilizan: electrones (en el caso de los convertidores termiónicos) o fotones (en el caso de los termofotovoltaicos). El problema fundamental de ambos dispositivos es que necesitan trabajar a temperaturas extremadamente elevadas para proporcionar suficiente densidad de potencia eléctrica. En los últimos años se han propuesto variaciones de estos dos tipos de convertidores básicos con el fin de aumentar la densidad de potencia eléctrica generada por el dispositivo. Por ejemplo, en la solicitud US201 10100430 se propone un convertidor de radiación lumínica en electricidad que combina los efectos fotovoltaico y termiónico. Aunque este dispositivo está concebido para convertir la luz solar, podría usarse igualmente para convertir calor en forma de radiación térmica incandescente. El convertidor de acuerdo a la solicitud US201 10100430 emplea una célula fotovoltaica para transformar directamente en electricidad los fotones más energéticos de la radiación y un convertidor termiónico para transformar los fotones menos energéticos, no aprovechables por la célula fotovoltaica. Para ello, el cátodo del convertidor termiónico se sitúa tras la célula fotovoltaica y se calienta mediante la absorción de los fotones menos energéticos de la radiación incidente, no absorbidos por la célula fotovoltaica. Si el cátodo alcanza una temperatura suficientemente alta, éste emitirá electrones hacia el ánodo. El ánodo, se dispone sobre la superficie no iluminada de la célula fotovoltaica, es decir entre la célula fotovoltaica y el cátodo, y colecta los electrones emitidos por el cátodo para cerrar el circuito. Este dispositivo es equivalente a una célula fotovoltaica de multi-unión con la salvedad de que los fotones de baja energía se transforman en electricidad de forma indirecta mediante un efecto termiónico, y no de forma directa mediante un efecto fotovoltaico. Debido a tratarse de una conversión indirecta (que requiere la generación intermedia de calor en el cátodo), la eficiencia de conversión de los fotones menos energéticos es considerablemente menor. Además, la densidad de potencia alcanzable por el convertidor sigue estando limitada por la cantidad de fotones emitidos por la fuente de radiación térmica (elemento externo al dispositivo). Para una temperatura y emisividad de fuente dada, este número de fotones está determinado por la ley de Plank e impone un límite a la transferencia de calor entre la fuente y el convertidor. Esto impone a su vez un límite a la potencia eléctrica producida por el convertidor. Una limitación similar ocurre en cualquier otro dispositivo termofotovoltaico (limitado por el número de fotones) o termiónico (limitado por el número de electrones). Among the solid state converters for the conversion of heat from very high temperature (above 1000 e C) into electricity, two types of devices stand out: thermo-photovoltaic and thermionic. These devices differ in the type of heat carriers they use: electrons (in the case of thermionic converters) or photons (in the case of thermo-photovoltaic). The fundamental problem of both devices is that they need to work at extremely high temperatures to provide sufficient electrical power density. In recent years, variations of these two types of basic converters have been proposed in order to increase the density of electrical power generated by the device. For example, in application US201 10100430 a converter of light radiation into electricity is proposed that combines the photovoltaic and thermionic effects. Although this device is designed to convert sunlight, it could also be used to convert heat in the form of incandescent thermal radiation. The converter according to US201 10100430 uses a photovoltaic cell to directly transform the most energetic photons of radiation into electricity and a thermionic converter to transform the less energetic photons, not usable by the photovoltaic cell. For this, the cathode of the thermionic converter is located behind the photovoltaic cell and is heated by absorbing the less energetic photons of the incident radiation, not absorbed by the photovoltaic cell. If the cathode reaches a sufficiently high temperature, it will emit electrons to the anode. The anode is placed on the unlit surface of the photovoltaic cell, that is, between the photovoltaic cell and the cathode, and collects the electrons emitted by the cathode to close the circuit. This device is equivalent to a multi-junction photovoltaic cell with the proviso that low energy photons are indirectly transformed into electricity by a thermionic effect, and not Direct form through a photovoltaic effect. Because it is an indirect conversion (which requires intermediate heat generation at the cathode), the conversion efficiency of the less energetic photons is considerably lower. In addition, the power density attainable by the converter remains limited by the amount of photons emitted by the source of thermal radiation (external element to the device). For a given source temperature and emissivity, this number of photons is determined by Plank's law and imposes a limit on the heat transfer between the source and the converter. This in turn imposes a limit on the electrical power produced by the converter. A similar limitation occurs in any other thermo-photovoltaic device (limited by the number of photons) or thermionic (limited by the number of electrons).
Por lo tanto, el objeto de esta invención es la de lograr un dispositivo capaz de extraer una mayor potencia de la fuente térmica y al mismo tiempo realizar una conversión directa del calor en electricidad con el fin último de aumentar la densidad de potencia eléctrica generada por el dispositivo. Therefore, the object of this invention is to achieve a device capable of extracting greater power from the thermal source and at the same time making a direct conversion of heat into electricity in order to increase the density of electrical power generated by the device.
RESUMEN DE LA INVENCIÓN SUMMARY OF THE INVENTION
El objeto de la presente invención es el de proporcionar un convertidor termiónico- fotovoltaico para la conversión directa del calor de alta temperatura en electricidad, que permita aumentar la densidad de potencia eléctrica extraída del foco térmico. The object of the present invention is to provide a thermo-photovoltaic converter for the direct conversion of high temperature heat into electricity, which allows to increase the density of electrical power extracted from the thermal source.
Para ello, la invención comprende un emisor (1 ), un colector (2) de electrones y una célula fotovoltaica (3). El emisor se calienta directamente mediante la fuente térmica (luz solar, combustión, reacción nuclear, etc.) gracias a la radiación recibida en una primera superficie (1 .1 ). Consecuentemente, el emisor emite fotones (4) y electrones (5) simultáneamente por una segunda superficie opuesta a la primera (1 .2). La emisión de fotones depende de la temperatura y la emisividad de dicha superficie, mientras que la emisión de electrones depende de la temperatura y de la función de trabajo. El colector (2) se encarga de colectar los electrones (5) y producir corriente eléctrica. Dicho colector debe ser trasparente, al menos parcialmente, para dejar paso a la radiación lumínica emitida por el emisor hacia la célula fotovoltaica (3). Dicha célula produce electricidad a partir de los fotones (4) emitidos por el emisor que no han sido absorbidos por el colector. La ventaja de este sistema híbrido, con respecto a US201 10100430, es que se consigue extraer una mayor cantidad de calor de la fuente térmica y por tanto es posible aumentar la densidad de potencia eléctrica. Esto se debe a que, a diferencia de US201 10100430, se utilizan dos tipos de portadores térmicos (electrones y fotones) para transferir la potencia calorífica de la fuente (en contacto íntimo con el emisor) al convertidor. Es importante destacar que en US201 10100430, los electrones se generan de forma indirecta mediante la absorción de fotones provenientes de la fuente térmica. La clave es que en este caso los electrones y los fotones se emiten directamente de la fuente térmica, y por tanto permiten extraer mucha más potencia calorífica de ésta. Además, y también a diferencia de US201 10100430, los fotones y electrones provenientes de la fuente térmica son trasformados directamente en electricidad por el convertidor: los fotones mediante un efecto fotovoltaico y los electrones mediante un efecto termiónico. Ambos efectos permiten aumentar la densidad de potencia eléctrica y la eficiencia del dispositivo. En un ejemplo de realización el colector y la célula son dos elementos independientes, pero el colector también puede estar depositado sobre un sustrato y/o sobre la propia célula. For this, the invention comprises an emitter (1), an electron collector (2) and a photovoltaic cell (3). The emitter is heated directly by the thermal source (sunlight, combustion, nuclear reaction, etc.) thanks to the radiation received on a first surface (1 .1). Consequently, the emitter emits photons (4) and electrons (5) simultaneously on a second surface opposite the first (1 .2). The emission of photons depends on the temperature and the emissivity of said surface, while the emission of electrons depends on the temperature and the work function. The collector (2) is responsible for collecting the electrons (5) and producing electric current. Said collector must be transparent, at least partially, to give way to the light radiation emitted by the emitter towards the photovoltaic cell (3). Said cell produces electricity from the photons (4) emitted by the emitter that have not been absorbed by the collector. The advantage of this hybrid system, with respect to US201 10100430, is that it is possible to extract a greater amount of heat from the thermal source and therefore it is possible to increase the density of electrical power. This is because, unlike US201 10100430, two types of thermal carriers are used (electrons and photons) to transfer the heat output of the source (in intimate contact with the emitter) to the converter. Importantly, in US201 10100430, electrons are generated indirectly by absorbing photons from the thermal source. The key is that in this case electrons and photons are emitted directly from the thermal source, and therefore allow to extract much more heat power from it. In addition, and also unlike US201 10100430, photons and electrons from the thermal source are directly transformed into electricity by the converter: photons through a photovoltaic effect and electrons through a thermionic effect. Both effects allow to increase the electrical power density and the efficiency of the device. In an exemplary embodiment, the collector and the cell are two independent elements, but the collector can also be deposited on a substrate and / or on the cell itself.
BREVE DESCRIPCIÓN DE LAS FIGURAS BRIEF DESCRIPTION OF THE FIGURES
Con objeto de ayudar a una mejor comprensión de las características de la invención y para complementar esta descripción, se acompañan como parte integrante de la misma las siguientes figuras, cuyo carácter es ilustrativo y no limitativo: La Fig. 1 muestra un convertidor híbrido termiónico-fotovoltaico de acuerdo a la invención, con conexión independiente de cada elemento, en el que el colector (2) es en un elemento físicamente separado del emisor (1 ) y de la célula fotovoltaica (3). El emisor (1 ) emite fotones (4) y electrones (5) hacia el colector (2). El colector absorbe los electrones y deja pasar los fotones a la célula fotovoltaica, dónde éstos se convierten en electricidad. El convertidor termiónico se conecta al exterior mediante las terminales (6) y (7), mientras que la célula fotovoltaica se conecta al exterior mediante las terminales (8) y (9). In order to help a better understanding of the features of the invention and to complement this description, the following figures are attached as an integral part thereof, whose character is illustrative and not limiting: Fig. 1 shows a thermionic hybrid converter- Photovoltaic according to the invention, with independent connection of each element, in which the collector (2) is in an element physically separated from the emitter (1) and the photovoltaic cell (3). The emitter (1) emits photons (4) and electrons (5) to the collector (2). The collector absorbs the electrons and lets the photons into the photovoltaic cell, where they become electricity. The thermionic converter is connected to the outside through terminals (6) and (7), while the photovoltaic cell is connected to the outside through terminals (8) and (9).
La Fig. 2 muestra otra posible realización en la que el colector se deposita directamente sobre la célula fotovoltaica, quedando ambos conectados eléctricamente. Fig. 2 shows another possible embodiment in which the collector is deposited directly on the photovoltaic cell, both being electrically connected.
La Fig. 3 muestra una realización similar a la anterior, a diferencia de que la corriente se extrae del dispositivo a través de dos únicas terminales (6) y (9). En este caso, la corriente que circula entre el emisor (1 ) y el colector (2) debe de ser la misma que la que circula entre los terminales positivo (3.1 ) y negativo (3.2) de la célula fotovoltaica. DESCRIPCIÓN DETALLADA Fig. 3 shows an embodiment similar to the previous one, unlike the current is drawn from the device through only two terminals (6) and (9). In this case, the current flowing between the emitter (1) and the collector (2) must be the same as that between the positive (3.1) and negative (3.2) terminals of the photovoltaic cell. DETAILED DESCRIPTION
El emisor (1 ) comprende un material refractario. En una posible realización, el emisor puede fabricarse mediante metales de alto punto de fusión (superior a 1700eC) y una presión de vapor relativamente baja (<10~9 atm a la temperatura de trabajo), como por ejemplo tungsteno, molibdeno, tántalo o platino. También se puede utilizar una atmósfera de cesio ionizado que ocupe el volumen existente entre el emisor y el colector, de forma que el cesio adsorbido en la superficie (1 .2) reduzca considerablemente la función de trabajo de dicha superficie y se facilite la emisión de electrones. En otra posible realización, el emisor (1 ) se fabrica utilizando un substrato refractario cuya única finalidad es la de proporcionar soporte mecánico y trasferir el calor de la superficie (1 .1 ) a la (1 .2). Para ello se pueden emplear, por ejemplo, materiales cómo el carburo de silicio o el grafito. En este caso, en la superficie (1 .2) del emisor se depositará una capa metálica para favorecer la emisión de electrones. Al igual que en la primera realización, es ventajoso emplear una atmósfera de cesio para reducir la función de trabajo del emisor. Cuando la fuente térmica empleada para calentar el emisor por su superficie (1 .1 ) es radiación lumínica, como por ejemplo luz solar, es ventajoso fabricar el emisor mediante un semiconductor, de forma que tenga lugar el efecto conocido como PETE ("photon enhanced thermionic emission", o emisión termiónica estimulada por fotones), y por lo tanto se facilite la emisión termiónica a temperaturas de emisor menores. The emitter (1) comprises a refractory material. In one possible embodiment, the emitter can be manufactured using high melting metals (above 1700 e C) and a relatively low vapor pressure (<10 ~ 9 atm at the working temperature), such as tungsten, molybdenum, tantalum or platinum. It is also possible to use an atmosphere of ionized cesium that occupies the volume between the emitter and the collector, so that the cesium adsorbed on the surface (1 .2) considerably reduces the work function of said surface and facilitates the emission of electrons In another possible embodiment, the emitter (1) is manufactured using a refractory substrate whose sole purpose is to provide mechanical support and transfer heat from the surface (1 .1) to (1 .2). For this, materials such as silicon carbide or graphite can be used, for example. In this case, a metallic layer will be deposited on the surface (1 .2) of the emitter to favor the emission of electrons. As in the first embodiment, it is advantageous to employ a cesium atmosphere to reduce the work function of the emitter. When the thermal source used to heat the emitter by its surface (1 .1) is light radiation, such as sunlight, it is advantageous to manufacture the emitter by means of a semiconductor, so that the effect known as PETE ("photon enhanced" thermionic emission ", or thermionic emission stimulated by photons), and therefore the thermionic emission at lower emitter temperatures is facilitated.
El colector (2) debe tener una función de trabajo reducida (del orden o menor que 1 .5 eV) y menor que la del emisor (para favorecer la colección de los electrones emitidos por el emisor) y una transmitancia óptica elevada (del orden o mayor al 70%, para permitir el paso de fotones del emisor a la célula fotovoltaica). Esta última característica debe cumplirse al menos en un rango espectral coincidente con parte de la respuesta espectral de la célula fotovoltaica (3). Para fabricar este colector se puede utilizar una de las siguientes configuraciones: Primero, una capa metálica muy fina (10-100nm) depositada sobre un substrato (bien sea la célula misma u otro soporte, como un cristal de cuarzo), que permita el paso de luz y a la vez tenga un comportamiento metálico que permita la colección eficiente de los electrones. Segundo, una lámina en forma de malla metálica que permita el paso de luz y al mismo tiempo permita la colección de electrones, que son direccionados selectivamente hacia las líneas metálicas de dicha malla. Tercero, semiconductores de ancho de banda elevado (> 1 .4 eV) y con afinidad electrónica reducida o incluso negativa (desde 0.5 eV a -2 eV), cómo por ejemplo GaN (bandgap de 3.2eV) adsorbido con cesio (que le confiere una afinidad electrónica de 0.5eV). Estos materiales no absorben los fotones de baja energía y al mismo tiempo facilitan la colección de electrones debido a tener una función de trabajo reducida. Cuarto, óxidos transparentes conductores, como por ejemplo el óxido de indio-estaño (ITO) o el óxido de tungsteno. Un experto en la materia reconocerá sin embargo que existen múltiples configuraciones posibles para el colector que van, desde una simple lámina o malla metálica, hasta la configuración arriba descrita. The collector (2) must have a reduced working function (of the order or less than 1 .5 eV) and less than that of the emitter (to favor the collection of electrons emitted by the emitter) and a high optical transmittance (of the order or greater than 70%, to allow the photons from the emitter to the photovoltaic cell). This last characteristic must be fulfilled at least in a spectral range coinciding with part of the spectral response of the photovoltaic cell (3). To make this collector one of the following configurations can be used: First, a very thin metallic layer (10-100nm) deposited on a substrate (either the cell itself or another support, such as a quartz crystal), which allows the passage of light and at the same time have a metallic behavior that allows the efficient collection of electrons. Second, a sheet in the form of a metallic mesh that allows the passage of light and at the same time allows the collection of electrons, which are selectively directed towards the metallic lines of said mesh. Third, semiconductors of high bandwidth (> 1 .4 eV) and with reduced or even negative electronic affinity (from 0.5 eV to -2 eV), such as GaN (3.2eV bandgap) adsorbed with cesium (which gives it an electronic affinity of 0.5eV). These materials do not absorb low energy photons and at the same time facilitate the collection of electrons due to having A reduced work function. Fourth, conductive transparent oxides, such as indium tin oxide (ITO) or tungsten oxide. A person skilled in the art will recognize, however, that there are multiple possible configurations for the collector ranging from a simple sheet or metal mesh, to the configuration described above.
La célula fotovoltaica (3) puede fabricarse empleando, al menos, un material semiconductor (por ejemplo Silicio, GaAs, Germanio, GaSb InGaAs, InGaAsSb, etc.) con el ancho de banda óptimo para el espectro de emisión lumínica del emisor (dependiente de la temperatura de éste) y formando al menos una unión p/n (cátodo/ánodo) para realizar los contactos selectivos de electrones y huecos generados internamente en el material semiconductor. Dicha célula tendrá al menos dos contactos eléctricos, uno positivo (cátodo) y otro negativo (ánodo) y podrá incorporar en su superficie trasera (3.2) un espejo que devuelva al emisor aquellos fotones no absorbidos por la célula, de forma que se reduzca la cantidad de calor a disipar en dicha célula y a su vez se aumente la eficiencia del convertidor. The photovoltaic cell (3) can be manufactured using at least one semiconductor material (for example Silicon, GaAs, Germanium, GaSb InGaAs, InGaAsSb, etc.) with the optimum bandwidth for the emitter's light emission spectrum (dependent on its temperature) and forming at least one p / n junction (cathode / anode) to make the selective contacts of electrons and holes generated internally in the semiconductor material. Said cell will have at least two electrical contacts, one positive (cathode) and the other negative (anode) and may incorporate in its rear surface (3.2) a mirror that returns to the emitter those photons not absorbed by the cell, so as to reduce the amount of heat to dissipate in said cell and in turn the efficiency of the converter is increased.
El colector (2) puede ser un elemento independiente (Fig.1 ), estar depositado directamente sobre la célula fotovoltaica (Fig.2 y Fig.3) o estar depositado sobre un sustrato trasparente (como por ejemplo un vidrio o cuarzo). En el último caso, dicho sustrato podría, a su vez, colocarse sobre la célula fotovoltaica (Fig.2 y Fig.3) o colocarse de forma independiente, separado de dicha célula por vacío o una atmósfera controlada (Fig.1 ). En cualquiera de los casos, el colector (ánodo del convertidor termiónico) podría conectarse eléctricamente al cátodo de la célula fotovoltaica, quedando ambos elementos conectados en serie, de forma que la corriente se extrae del convertidor entre las terminales del emisor (6) (cátodo del convertidor termiónico) y el ánodo de la célula fotovoltaica (9) (Fig.3). Igualmente, se pueden realizar conexiones independientes tanto al colector (7) (ánodo del convertidor termiónico) como al cátodo de la célula fotovoltaica (8), para extraer la corriente por dos circuitos independientes (Fig.1 y Fig.2). La ventaja de esta última configuración es que ambos dispositivos pueden polarizarse en sus respectivos puntos de máxima potencia de forma independiente. Por el contrario, en el caso de que sólo existan dos terminales, es necesario un ajuste en corriente entre ambos dispositivos, lo cual impide, en la mayoría de los casos, la polarización de cada dispositivo en su punto de máxima potencia. La ventaja de una configuración de dos terminales es que su fabricación resulta más sencilla y por lo tanto tiene un mayor potencial de reducción de costes. Alternativamente a la incorporación de una atmósfera de cesio, el colector puede colocarse a una distancia micrométrica de la superficie (1 .2) del emisor (1 ) para favorecer la trasferencia de electrones entre ambos elementos. En este caso, es ventajoso que exista vacío entre emisor y colector y que el colector esté depositado directamente en la superficie de la célula fotovoltaica, para de este modo aprovechar los fotones que se trasmiten del emisor al colector de forma evanescente (efectos túnel). The collector (2) can be an independent element (Fig. 1), be deposited directly on the photovoltaic cell (Fig. 2 and Fig. 3) or be deposited on a transparent substrate (such as glass or quartz). In the latter case, said substrate could, in turn, be placed on the photovoltaic cell (Fig. 2 and Fig. 3) or placed independently, separated from said cell by vacuum or a controlled atmosphere (Fig. 1). In either case, the collector (anode of the thermionic converter) could be electrically connected to the cathode of the photovoltaic cell, both elements being connected in series, so that the current is drawn from the converter between the emitter terminals (6) (cathode of the thermionic converter) and the anode of the photovoltaic cell (9) (Fig. 3). Likewise, independent connections can be made both to the collector (7) (anode of the thermionic converter) and to the cathode of the photovoltaic cell (8), to extract the current through two independent circuits (Fig. 1 and Fig. 2). The advantage of this last configuration is that both devices can polarize at their respective maximum power points independently. On the contrary, in the case that there are only two terminals, a current adjustment between both devices is necessary, which prevents, in most cases, the polarization of each device at its maximum power point. The advantage of a two-terminal configuration is that its manufacturing is simpler and therefore has a greater potential for cost reduction. Alternatively to the incorporation of a cesium atmosphere, the collector can be placed at a micrometric distance from the surface (1 .2) of the emitter (1) to favor the electron transfer between both elements. In this case, it is advantageous that there is a vacuum between emitter and collector and that the collector is deposited directly on the surface of the photovoltaic cell, in order to take advantage of the photons that are transmitted from the emitter to the collector in an evanescent manner (tunnel effects).
DESCRIPCIÓN DE UNA REALIZACIÓN PREFERIDA DESCRIPTION OF A PREFERRED EMBODIMENT
En una realización preferida, el emisor (1 ) se fabrica en tungsteno. El volumen existente entre el emisor y el colector se rellena de un gas de cesio ionizado, de forma que la función de trabajo del tungsteno queda reducida mediante la adsorción de cesio en la superficie, alcanzando un valor en el orden de los 1 .7 eV. En este caso, se necesitará de una fuente de cesio externa para reponer el cesio consumido de la superficie del emisor. In a preferred embodiment, the emitter (1) is manufactured in tungsten. The volume between the emitter and the collector is filled with an ionized cesium gas, so that the working function of tungsten is reduced by adsorption of cesium on the surface, reaching a value in the order of 1 .7 eV . In this case, an external cesium source will be needed to replace the consumed cesium from the emitter surface.
El colector (2) es una fina lámina de óxido de tungsteno, depositada en un sustrato de cuarzo. El óxido de tungsteno, al ser adsorbido con el cesio existente en la atmósfera, alcanza funciones de trabajo del orden de 0.75 eV. El espesor de esta capa (entre 1 y 100nm) es suficientemente pequeño, para que la luz pueda atravesarla y alcanzar la célula fotovoltaica (3). El sustrato de cuarzo, que contiene el colector, se deposita directamente sobre la célula fotovoltaica empleando una silicona trasparente para garantizar la continuidad de índice de refracción entre el sustrato de cuarzo y la superficie de la célula fotovoltaica. En esta configuración, la cara del sustrato de cuarzo que contiene el colector debe quedar orientada hacia el emisor. The collector (2) is a thin sheet of tungsten oxide, deposited on a quartz substrate. Tungsten oxide, when adsorbed with cesium in the atmosphere, achieves work functions of the order of 0.75 eV. The thickness of this layer (between 1 and 100nm) is small enough, so that light can pass through it and reach the photovoltaic cell (3). The quartz substrate, which contains the collector, is deposited directly on the photovoltaic cell using a transparent silicone to guarantee the continuity of refractive index between the quartz substrate and the surface of the photovoltaic cell. In this configuration, the face of the quartz substrate containing the collector must face the emitter.
La célula fotovoltaica (3) se fabrica partiendo de un sustrato de GaSb en el que se forma una unión p/n. El GaSb permite absorber fotones con energías por encima de los 0.7 eV y por tanto se ajusta a los espectros de emisión correspondientes a las temperaturas de trabajo del emisor (1 ), de entre 1000eC y 1800eC. La zona tipo-p (cátodo) se sitúa en la capa frontal de dicha célula (3.1 ) para facilitar la eventual conexión entre el terminal positivo de la célula (3.1 ) y el colector (2) y de esta forma conectar en serie el convertidor termiónico con el fotovoltaico. En su cara posterior (3.2) la célula fotovoltaica dispone de un reflector que devuelva los fotones no absorbidos por la célula al emisor. Este reflector puede fabricarse mediante una estructura de capas dieléctricas o mediante un metal especular muy reflectante, como por ejemplo, el oro. En esta configuración se pueden realizar cuatro contactos eléctricos (Fig.1 ): en el emisor (6), el colector (7), el cátodo (8) y el ánodo de la célula fotovoltaica (9), de forma que la corriente se extrae del dispositivo a través de dos circuitos independientes: uno formado por las terminales de emisor (6) y colector (7), y otro formado por las terminales de cátodo (8) y ánodo (9) de la célula fotovoltaica. A la vista de esta descripción y figura, el experto en la materia podrá entender que la invención ha sido descrita según algunas realizaciones preferentes de la misma, pero que múltiples variaciones pueden ser introducidas en dichas realizaciones preferentes, sin salir del objeto de la invención tal y como ha sido reivindicada. The photovoltaic cell (3) is manufactured from a GaSb substrate in which a p / n junction is formed. The GaSb allows photons to be absorbed with energies above 0.7 eV and therefore conforms to the emission spectra corresponding to the emitter's working temperatures (1), between 1000 e C and 1800 e C. The p-type zone (cathode) is located in the front layer of said cell (3.1) to facilitate the eventual connection between the positive terminal of the cell (3.1) and the collector (2) and thus connect in series the thermionic converter with the photovoltaic. On its rear face (3.2) the photovoltaic cell has a reflector that returns the photons not absorbed by the cell to the emitter. This reflector can be manufactured by means of a structure of dielectric layers or by a highly reflective specular metal, such as gold. In this configuration four electrical contacts can be made (Fig. 1): in the emitter (6), the collector (7), the cathode (8) and the anode of the photovoltaic cell (9), so that the current it is extracted from the device through two independent circuits: one formed by the emitter (6) and collector (7) terminals, and another formed by the cathode (8) and anode (9) terminals of the photovoltaic cell. In view of this description and figure, the person skilled in the art may understand that the invention has been described according to some preferred embodiments thereof, but that multiple variations can be introduced in said preferred embodiments, without departing from the object of the invention such and as claimed.

Claims

REIVINDICACIONES
Convertidor híbrido termiónico-fotovoltaico para la conversión directa de calor en electricidad que comprende: un emisor de electrones y fotones fabricado en un material refractario (1 ), un colector de electrones (2) transparente a la radiación fotónica en la longitud de onda emitida por el emisor y una célula fotovoltaica (3), estando estos elementos dispuestos de manera que el emisor tiene una primera superficie(1 .1 ) destinada a ser orientada hacia una fuente térmica y una segunda superficie opuesta a la primera (1 .2) que emite electrones y fotones, el colector se sitúa frente a esta segunda superficie de manera que recibe los electrones emitidos por el emisor (1 ) y la célula (3) se sitúa tras el colector (2) de manera que recibe los fotones que atraviesan dicho colector. Hybrid thermionic-photovoltaic converter for the direct conversion of heat into electricity comprising: an electron and photon emitter made of a refractory material (1), an electron collector (2) transparent to photonic radiation at the wavelength emitted by the emitter and a photovoltaic cell (3), these elements being arranged so that the emitter has a first surface (1 .1) intended to be oriented towards a thermal source and a second surface opposite the first (1 .2) that emits electrons and photons, the collector is placed in front of this second surface so that it receives the electrons emitted by the emitter (1) and the cell (3) is located behind the collector (2) so that it receives the photons that cross said manifold.
Convertidor según la reivindicación 1 caracterizado porque el colector (2) y la célula (3) son dos elementos independientes. Converter according to claim 1 characterized in that the collector (2) and the cell (3) are two independent elements.
Convertidor según la reivindicación 1 caracterizado porque el colector (2) está depositado sobre un sustrato y/o sobre la célula (3). Converter according to claim 1 characterized in that the collector (2) is deposited on a substrate and / or on the cell (3).
Convertidor según las reivindicaciones 1 o 2 caracterizado porque el colector (2) comprende una capa metálica de 10 a 100nm de espesor depositada sobre un substrato, una lámina en forma de malla, una capa de semiconductores de ancho de banda mayor que 1 .4 eV y con afinidad electrónica desde 0.5 eV a -2 eV y una capa de un óxido transparente conductor. Converter according to claims 1 or 2, characterized in that the collector (2) comprises a metal layer 10 to 100nm thick deposited on a substrate, a mesh-shaped sheet, a semiconductor layer with a bandwidth greater than 1 .4 eV and with electronic affinity from 0.5 eV to -2 eV and a layer of a conductive transparent oxide.
Convertidor según cualquiera de las reivindicaciones anteriores, caracterizado porque en el espacio entre el emisor y el colector existe una atmósfera de gas de cesio ionizado. Converter according to any of the preceding claims, characterized in that in the space between the emitter and the collector there is an atmosphere of ionized cesium gas.
Convertidor según cualquiera de las reivindicaciones 1 -4 donde el colector (2) está situado a una distancia micrométrica de la segunda superficie (1 .2) del emisor (1 ). Converter according to any of claims 1 -4 wherein the collector (2) is located at a micrometric distance from the second surface (1 .2) of the emitter (1).
Convertidor según cualquiera de las reivindicaciones anteriores caracterizado porque comprende terminales de conexión del emisor (6), el colector (7), el cátodo (8) y el ánodo de la célula fotovoltaica (9), de forma que la corriente se puede extraer del dispositivo a través de dos circuitos independientes: uno formado por las terminales de emisor (6) y colector (7), y otro formado por las terminales de cátodo (8) y ánodo (9) de la célula fotovoltaica. Converter according to any of the preceding claims characterized in that it comprises connection terminals of the transmitter (6), the collector (7), the cathode (8) and the anode of the photovoltaic cell (9), so that the current can be extracted from the device through two independent circuits: one formed by the terminals emitter (6) and collector (7), and another formed by the cathode (8) and anode (9) terminals of the photovoltaic cell.
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