FR3019936A1 - PHOTO-THERMO-VOLTAIC CELL WITH PLASMA GENERATOR BY MICROONDE RESONANCE - Google Patents

Abstract

A cell (100) for generating an electric current by converting thermal and photonic energies into electrical energy, said cell (100) comprising a photo-thermo-voltaic converter (108), said converter comprising an emitting cathode (122) semiconductor device adapted to be illuminated by electromagnetic waves (R) to emit electrons, and a collector anode (124) adapted to collect the electrons emitted by the emitting cathode (122), wherein the emitting cathode (122) ) and the collecting anode (124) define between them an electron transfer space (129). The cell (100) is characterized in that the photo-thermo-voltaic converter (108) comprises a microwave resonance plasma generator (120) adapted to generate a plasma in the electron transfer space (129). ). Preferred application for converting solar energy into electrical energy.

Description

The present invention relates to a cell for generating an electric current by converting thermal and photonic energies into electrical energy, said cell comprising a photo-thermo-voltaic converter, the said cell being converter comprising a semiconductor emitter cathode adapted to be illuminated by electromagnetic waves to emit electrons and a collector anode adapted to collect the electrons emitted by the emitting cathode, in which the emitting cathode and the collector anode define between they have an electron transfer space. Such a cell is shown in FIG. 6a of US 2010/0139771 A1, hereinafter referred to as "Schwede document". This known electrical current generation cell is based on the principle of thermoelectronic emission with photonic stimulation, also called, by misuse of language, "photon-enhanced thermionic emission" (Photon Enhanced Thermionic Emission). PETE).

This known PETE cell directly transforms both the visible part and the infrared part of the solar radiation into electrical current. This type of cell suffers from space charge effects limiting its performance. Indeed, the accumulation of electrons emitted by the cathode of the cell and moving towards the anode create an electric field and thus a potential barrier opposing the passage of additional electrons from the cathode to the anode. US 5,994,638 describes two ways to prevent the formation of the potential barrier. The first solution is to bring the cathode and the anode as close as possible. The second solution is to introduce a cesium plasma between the cathode and the anode. Cesium is chosen because of its particularly low ionization potential which allows ionization by simple thermal contact with the cathode at very high temperature. Cesium also lowers the output work of the anode and increases the anode-cathode voltage of the device. The first solution has the disadvantage of limiting the temperature difference achievable between the two electrodes and therefore the efficiency of the cell.

The second solution is not very viable because of the aggressiveness of the cesium plasma, which considerably reduces the life of the cell. An object of the present invention is therefore to propose a cell of the type defined above having a good efficiency, in particular by eliminating space charge effect problems in an innovative way. These include efficient and low energy energy ionization of the interelectrode gas when a relatively low temperature cathode is employed.

This object is achieved in that the photo-thermo-voltaic converter comprises a microwave resonance plasma generator adapted to generate a plasma in the electron transfer space. By providing a microwave resonance plasma generator in the cell, it becomes possible to create a plasma between the electrodes with a substantially inert gas. The created plasma neutralizes the space charge and does not degrade the electrodes. Thus, during the operation of the cell, the anode can collect a larger number of electrons emitted by the cathode, and the life of the cell is significantly increased compared to the known use of a cesium plasma.

Preferably, the plasma generator integrated in the cell according to the invention is based on the microwave resonant structure described in the patent application FR 2 958 187 A1 filed by the applicant. The inventor of the present invention has surprisingly discovered that the microwave resonant structure of FR 2 958 187 A1 developed for the production of chemical species can be suitably applied to PETE cells. According to preferred embodiments, the cell according to the invention comprises one, several or all of the following characteristics, according to all technically possible combinations: the plasma generator comprises a microwave resonant element of the quarter-antenna type; wave and a ground surface vis-à-vis the resonant element; the resonant element terminates in a high-frequency electrode, preferably at least partially transparent to solar radiation; the high frequency electrode is in the form of a comb or a grid; the high frequency electrode and the collector anode are combined; the emitting cathode is disposed on or within the ground surface; the emitting cathode is separated from the ground surface by a thermal insulator; the photo-thermo-voltaic converter further comprises an auxiliary electrode, preferably in the form of a ring, for controlling the electric potential in the electron transfer space; the photo-thermo-voltaic converter further comprises an electron-guiding magnet passing through the electron transfer space towards the collecting anode; the magnet is arranged on the side of the emitting cathode remote from the electron transfer space; the emitting cathode comprises a semiconductor substrate covered with a semiconductor layer capable of emitting electrons by thermal and photonic excitation, in particular a layer of gallium nitride, and an infrared radiation absorption layer; - The cell defines a chamber accommodating the photo-thermo-voltaic converter, said chamber being filled with an inert gas or substantially inert at a pressure between 0.1 and 10 kPa. The invention also covers an electricity generator comprising a cell as defined above and a solar radiation concentrator, in particular a Fresnel lens or a parabolic mirror, in which the cell is provided with an illumination window of the transmitter cathode, and wherein the cell is arranged with respect to the concentrator so that the concentrator can illuminate the emitter cathode at its focal point through said illumination window. The invention also relates to a method for generating an electric current by insolation of a cell as defined above, comprising the steps of: a) establishing a working pressure between 0.1 and 10 kPa in the cell ; b) introducing an inert or substantially inert gas into the electron transfer space; c) starting the plasma generator, thereby ionizing said gas in the electron transfer space; and d) exposing the emitting cathode to ultraviolet-visible-infrared radiation. According to preferred embodiments, the method according to the invention comprises one, several or all of the following characteristics, according to all the technically possible combinations: the step of extinguishing the plasma generator when the cell has reached a regime ionization of gas by electric current; the gas contains traces of vaporized cesium. The invention will be better understood on reading the description which follows, given solely by way of example and with reference to the appended drawings in which: FIG. 1 is an overall and side view of a cell generating an electric current according to the invention electrically connected to a load; FIG. 2 shows an exemplary embodiment of the high frequency electrode according to the invention; Figure 3 shows an example of the structure of the cathode of the cell according to the invention; Figure 4 is an enlarged view of detail A of Figure 1 according to a first embodiment of the invention; Figure 5 is a view similar to Figure 4, showing a second variant of the invention; and Fig. 6 is a view of an array of cells with solar radiation concentrators forming an electrical generator according to the invention. FIG. 1 shows a cell 100 for generating an electric current by converting thermal and photonic energies into electrical energy according to one embodiment of the invention.

The cell 100 creates an electric current based on the photon enhanced thermoelectronic emission principle. This principle is explained in detail in the Schwede document. A cathode emits electrons by heating and by absorption of photons that are collected by an anode. Thus, an electric current flows between the cathode and the anode. The cell 100 can therefore be called photo-thermo-voltaic cell because it creates a voltage at its terminals by thermal and photonic excitation of electrons. In contrast, the voltage across a conventional photovoltaic cell is solely due to the photon excitation of the electrons. Cell 100 also differs from simple known thermo-electronic cells, as described for example in US 4,667,126. Indeed, in the cell 100, the electric current is not only obtained by heating but also by photon excitation of the electrons in the cathode. With reference to FIG. 1, the cell 100 is provided with a positive terminal 102 and a negative terminal 104. A load L is connected to the terminals 102, 104. The cell 100 comprises an enclosure 106 accommodating a photothermal converter 108. The enclosure 106 has a generally cylindrical shape. Preferably, the enclosure 106 is made of metal. The bottom of the enclosure 106 is designated 110, and its gas inlet 112. The bottom 106 and the inlet 112 are at opposite ends of the enclosure 106. The orifice intake 112 is provided with a stop valve 114 for closing and opening the inlet port 112. Alternatively, the inlet port 112 is sealed once the cell 100 is operational. The enclosure 106 has a window 116 arranged in one of its walls. The window 116 is transparent to ultraviolet radiation, visible and infrared, and in particular to solar radiation, as shown by the arrow R. The window 116 may for example be made from magnesium fluoride or calcium fluoride.

The chamber 106 defines a chamber 118 in which the photo-thermo-voltaic converter 108 is located. The chamber 118 is filled with an inert or substantially inert gas G. Preferably, a gas G is selected as the gas. ionizes easily, that is to say that can be transformed into plasma with a limited energy input, and that does not attack the components of the photo-thermo-voltaic converter 108, and in particular to its electrodes. Thus, the ionization potential of the gas G is preferably less than or equal to 15 eV. Argon, helium, xenon, hydrogen or a mixture of these gases are preferably used as gases G. The gas G can optionally include traces of cesium or vaporized mercury to initiate ionization by microwaves. waves.

The pressure P in the chamber 118 is preferably lower than the atmospheric pressure. P may especially be between 0.1 and 10 kPa. The photo-thermo-voltaic converter 108 comprises a plasma generator 120, an electron-emitting cathode 122 and an electron-collecting anode 124. The plasma generator 120 essentially corresponds to the resonant microwave structure described in the document FR. 2 958 187 A1 filed by the applicant, and in particular represented in Figure 3 of this document. For a detailed description of the elements of the plasma generator 120, reference is made to this document. Thus, the plasma generator 120 comprises a microwave resonant element of quarter-wave antenna type 126 and a mass surface 128 vis-à-vis the resonant element 126. The quarter wave antenna 126 and the mass surface 128 is preferably of copper. Advantageously, the copper quarter wave antenna 126 is covered with a gold layer in order to increase the surface conductivity and therefore its Q quality factor, thus minimizing the energy to be injected to create the plasma.

The quarter wave antenna 126 ends with a high frequency electrode 124. In the embodiment described here, the high frequency electrode is also the collector anode 124. In other words, the electrode high frequency and collector anode are merged. In a variant not shown, the collecting anode and the high frequency electrode may be two distinct elements. Preferably, the metal of the collecting anode is chosen so that its output work is small. This induces a stronger electromotive force of the cell 100. The high frequency electrode 124 is located below the window 116, between this window and the emitting cathode 122. Thus, the high frequency electrode 124 must be at least partially transparent to the radiation R from the window 116 so that at least a portion of this radiation R reaches the emitting cathode 122. For this purpose, the high frequency electrode 124 may take the form of a comb or a fork, as illustrated in Figure 2, with gaps 125. Alternatively, the high frequency electrode may be a grid, especially with an optical transparency of about 80%, this grid can be carried by a ring of comparable to that of the cathode emitter 122. In another variant, the high frequency electrode is a disk at least partially transparent to ultraviolet radiation, visible and infrared. 129 denotes the electron transfer space separating the emitting cathode 122 from the collector anode 124. Preferably, the extent of this space 129, that is to say the normal distance between the emitting cathode 122 and the collector anode 124 is of the order of a few millimeters. The emitting cathode 122 is shown in detail in FIG. 3. In this example, the emitting cathode 122 is fixed on the ground surface 128. Nevertheless, the emitting cathode 122 could also be disposed within the ground surface 128.

The emitting cathode 122 is separated from the ground surface 128 by a thermal insulator 130. This thermal insulator 130 is preferably a ceramic plate. The emitting cathode 122 here comprises three layers 132, 134 and 136. The first layer 132 rests on the thermal insulator 130. It is a layer 132 for absorbing infrared radiation. The absorption layer 132 may consist of a layer of metal oxide or carbon nanotubes with a very high thermal absorption coefficient (for example 98%). Preferably, the second layer 134 is a semiconductor substrate, for example sapphire. The substrate 134 is covered with the third layer 136. This third layer 136 is semiconductor and capable of emitting electrons by thermal and photonic excitation. The photo-thermo-emissive layer 136 is for example made from gallium nitride, gallium arsenide or silicon carbide, optionally doped, or nitrogen-doped diamond. Advantageously, the photo-thermo-emissive layer 136 is nanostructured. Nanostructures facilitate the emission of electrons. An example of nanostructures are nano-f salient ones. The Schwede document teaches several examples of adapted nanostructures, cf. Figures 9a to 9c, which are incorporated by reference in the present description. FIGS. 4 and 5 show in detail the zone A identified by dashed lines in FIG. 1. The plasma I can be distinguished between the emitting cathode 122 and the high frequency electrode 124. These two figures illustrate variants in which the electrode High frequency 124 and the collector anode are two separate components.

In these variants, the collector anode surrounds the emitting cathode 122 and the high frequency electrode 124. The collector anode is not visible in FIGS. 4 and 5 because it is located in front of and behind the plane represented by these figures. The collecting anode thus comprises two surfaces perpendicular to the emitting cathode 122 and to the high frequency electrode 124. The collector anode binds two opposite sides of the electron transfer space 129. The emitting cathode 122 and the High frequency electrode 124 bound two opposite sides of the electron transfer space 129 different from those bounded by the collector anode. In addition, FIGS. 4 and 5 show two additional optional devices 138, 140 of the photo-thermo-voltaic converter 108 serving to optimize the current generation by the cell 100. The first device is an auxiliary electrode 138, preferably in the form of a ring, electrical potential control in the electron transfer space 129. This auxiliary electrode 138 has an internal diameter substantially equal to the size of the emitting cathode 122. It is located in the electron transfer space 129, between the emitting cathode 122 and the high frequency electrode 124, preferably about equidistant between the emitting cathode 122 and the high frequency electrode 124. The auxiliary electrode 138 makes it possible to define the plasma potential between the anode and the cathode and thus to control and optimize the current of the cell, playing a role similar to the gate in a triode. The second device 140 is a single magnet (see FIG. 4) or a set of magnets (see FIG. 5) for guiding the electrons passing through the electron transfer space 129. The magnet (s) 140 creates (s) a magnetic field B in the electron transfer space 129 whose field lines are shown. Thanks to the magnetic field B, the electrons leaving the emitting cathode 122 are guided towards the collecting anode. According to the variant of Figure 4, a single permanent magnet 140 is arranged on the side of the emitting cathode 122 remote from the electron transfer space 129. In the variant of Figure 5, a second permanent magnet is disposed on the side of the high frequency electrode 124 remote from the electron transfer space 129. Thus, the two magnets 140 are vis-à-vis. More specifically, the two magnets 140 are arranged so that their magnetic poles of the same polarity are rotated towards each other. This results in a magnetic field B in the electron transfer space 129 capable of diverting the electrons towards the collecting anode.

One of the two magnets 140 is pierced with a hole 142 allowing the radiation R to pass towards the emitting cathode 122.

Thanks to the magnets 140, the collecting anode can be further away from the emitting cathode 122. Consequently, the temperature gradient between these two electrodes can be increased, which increases the efficiency of the cell 100. Advantageously, an electrical insulator such as that a mica sheet 144 is disposed near the high frequency electrode 124, between the latter and the emitting cathode 122. The mica sheet 144 masks the high frequency electrode 124 and prevents electrons in the electron transfer space 129 to reach the high frequency electrode 124 instead of the lateral collector anode. FIG. 6 shows an electricity generator 200 connected by its terminals 201 to a load L. The electricity generator 200 comprises a network 202 of electrical current generation cells 100 electrically connected in series. For the sake of simplicity, the speakers of cells 100 have been omitted. The cells 100 are of the type where the high frequency electrode 124 operates at the same time as a collector anode. The electricity generator 200 further comprises solar radiation concentrators 204. Each solar radiation concentrator 204 is associated with the emitting cathode 122 of a cell 100. Each cell 100 is arranged relative to its concentrator 204 in such a way as to that the concentrator 204 can illuminate the emitting cathode 122 at its focal point F. In Fig. 6, the concentrators 204 are Fresnel lenses. Alternatively, concentrators 204 may also be parabolic mirrors. Preferably, each Fresnel lens 204 has a surface area of about 1,000 cm 2, for example made in the form of a square with a side of a length of 30 cm. In a variant, an additional convergent lens 206, shown in dashed lines in FIG. 6, is added between the Fresnel lenses 204 and the cells 100.

The lenses 204, 206 are chosen in such a way as to obtain a radiation concentration factor of at least 300. The operation of the electricity generator 200 according to FIG. 6 will now be described. Firstly, microwaves are injected at a minimum. 450 MHz frequency in the plasma generators 120. Thus, the quarter-wave antennas 126 come into resonance which ionizes the gas G present in each chamber 118 at each electron transfer space 129. The resulting plasma I has a volume of about 1 cm3. Plasma I is in dynamic equilibrium where electrons are constantly torn from gas atoms to create free ions and electrons, and free electrons constantly recombine with ions as atoms of gas. The created plasma is generally neutral charge.

Preferably, the injected microwaves are modulated at the frequency of the alternating current distributed by the electrical networks, ie 50 Hz in Europe, in order to generate an adapted alternating current. The electricity generator 200 is then placed under solar radiation. The solar rays R are focused by the Fresnel lenses 204, and possibly the convex lenses 206, on the emitting cathodes 122. The solar rays R then pass through the high frequency electrodes 124 through the interstices 125. The solar rays R incident on each photo-thermo-emissive layer 136 is absorbed by it. More precisely, as illustrated in FIG. 1 of the Schwede document, the photons of the visible part of the radiation R excite the electrons of the photo-thermo-emissive layer 136 of the valence band towards the conduction band. At the same time, the infrared portion of the radiation R heats the emitting cathode 122 at a temperature of about 800 ° C. Thanks to the heating and the photonic excitation, electrons leave the photo-thermo-emissive layer 136 towards the electron transfer space 129. These electrons are caught by the collecting anode, that is to say the high frequency electrode 124. Thanks to the positive ion plasma I, the emitted electrons are prevented from generating a space charge between the collector anode 124 and the emitting cathode 122. Thus, the electrons pass through the plasma I which is for the cathode a simple conductive medium and reach unhindered the collector anode 124. Therefore, an electric current flows between the terminals 201 of the electric generator 200, and therefore through the load L. This is a alternating current of a frequency of 50 Hz, due to the modulation of microwaves injected into the plasma generators 120. However, alternatively, it is also possible to operate the electricity generator 200 so as to generate an electric current. continuous electric current or an alternating electric current at another frequency. Once the electric generator 200 operates in steady state, it is possible to extinguish the plasma generators 120. In fact, the ionization of the gas G is then maintained by the electric current flowing. The plasma generators 120 can therefore be used only as a starter of the electric generator 200, similar to the operation of the oxide cathodes in the fluorescent tubes where the cathode is extinguished after initiation of the electric discharge in the tube.

Claims (16)

CLAIMS 1. Cell (100) for generating an electric current by converting thermal and photonic energies into electrical energy, said cell (100) comprising a photo-thermo-voltaic converter (108), said converter comprising: a cathode semiconductor emitter (122) adapted to be illuminated by electromagnetic waves (R) to emit electrons; and a collecting anode (124) adapted to collect the electrons emitted by the emitting cathode (122); wherein the emitting cathode (122) and the collector anode (124) define therebetween an electron transfer space (129), the cell (100) being characterized in that the photothermal converter (108) comprises a microwave resonance plasma generator (120) adapted to generate a plasma in the electron transfer space (129). The cell (100) according to claim 1, wherein the plasma generator (120) comprises a quarter wave antenna type resonant element (126) and a ground surface (128) facing one another. -vis the resonant element. 3. Cell (100) according to claim 2, wherein the resonant element (126) terminates in a high frequency electrode (124), preferably at least partially transparent to solar radiation. 4. Cell (100) according to claim 3, wherein the high frequency electrode (124) has the shape of a comb or a grid. 5. Cell (100) according to any one of claims 3 or 4, wherein the high frequency electrode (124) and the collector anode are combined. 6. Cell (100) according to any one of claims 2 to 5, wherein the emitting cathode (122) is disposed on or within the ground surface (128). The cell (100) of claim 6, wherein the emitting cathode (122) is separated from the ground surface (128) by a thermal insulator (130). 8. The cell (100) according to any one of the preceding claims, wherein the photo-thermo-voltaic converter (108) further comprises an auxiliary electrode (138), preferably ring-shaped, potential control. in the electron transfer space (129). The cell (100) according to any one of the preceding claims, wherein the photo-thermo-voltaic converter (108) further comprises a magnet (140) for guiding electrons through the electron transfer space ( 129) to the collecting anode (124). 10. Cell (100) according to claim 9, the magnet (140) being arranged on the side of the emitting cathode (122) remote from the electron transfer space (129). 11. Cell (100) according to any one of the preceding claims, wherein the emitting cathode (122) comprises a semiconductor substrate (134) covered with a semiconductor layer (136) capable of emitting electrons by thermal and photonic excitation, in particular of a layer of gallium nitride, and an absorption layer (132) of infrared radiation. 12. Cell (100) according to any one of the preceding claims, the cell (100) defining an enclosure (106) accommodating the photo-thermo-voltaic converter (108), said enclosure (106) being filled with a gas inert or substantially inert (G) at a pressure between 0.1 and 10 kPa. 13. An electricity generator (200) comprising a cell (100) according to any one of the preceding claims and a solar radiation concentrator (204), in particular a Fresnel lens or a parabolic mirror, wherein the cell (100) ) is provided with a window (116) for illuminating the emitting cathode (122), and wherein the cell (100) is arranged with respect to the concentrator (204) so that the concentrator (204) can illuminating the emitting cathode (122) at its focal point (F) through said illumination window (116). 14. A method of generating an electric current by insolation of a cell (100) according to any one of the preceding claims, comprising the steps of: a) establishing a working pressure between 0.1 and 10 kPa in the cell (100); b) introducing an inert or substantially inert gas (G) into the electron transfer space (129); c) starting the plasma generator (120), thereby ionizing said gas (G) in the electron transfer space (129); and d) exposing the emitting cathode (122) to ultraviolet-visible infrared (R) radiation. The method of claim 14, further comprising the step of extinguishing the plasma generator (120) when the cell (100) has reached a gas ionization rate (G) by electric current. 16. A process according to claim 14 or 15, wherein the gas (G) contains traces of vaporized cesium.