FR2895392A1 - Production of hydrogen by water dissociation reaction from solar radiation in a treatment chamber, comprises focussing solar radiation for dissociating water vapor molecules and submitting photo catalytic target to focussed solar radiation - Google Patents

Abstract

The production of hydrogen by water dissociation reaction from solar radiation in a treatment chamber, comprises focussing solar radiation for dissociating water vapor molecules, submitting photo catalytic target to focussed solar radiation, dissociating hydrogen and oxygen from the water vapor molecules of a boundary layer in contact with the surfaces at the target, and selective extracting of the hydrogen and oxygen produced by selective extraction membrane. The dissociation reaction is carried out at an area pressure of 3-15 bars and at 300-1000[deg]C. The production of hydrogen by water dissociation reaction from solar radiation in a treatment chamber, comprises focussing solar radiation for dissociating water vapor molecules, submitting photo catalytic target to focussed solar radiation, dissociating hydrogen and oxygen from the water vapor molecules of a boundary layer in contact with the surfaces at the target, and selective extracting of the hydrogen and oxygen produced by selective extraction membrane. The dissociation reaction is carried out at an area pressure of 3-15 bars and at 300-1000[deg]C. A surface volume of the selective membrane with hydrogen and/or oxygen are defined to obtain an ionic current density of 1 A/cm 2>. The focussed rate of the solar radiation is 500-3000. The dissociation rate of the water vapor molecules of the boundary layer is increased by the application of an electric field between the selective permeable membranes for oxygen and hydrogen. An independent claim is included for an apparatus for the production of hydrogen.

Description

The invention relates to a method and an equipment for producing

  Hydrogen by dissociation of water vapor by means of concentrated solar energy. It applies more particularly but not exclusively, for applications in many areas:

  On board a space station or a satellite, hydrogen and oxygen may be used as a means of storing energy. The water on board such a station will circulate in a closed loop in a chain of user devices and can thus be used for several successive uses. This loop will necessarily include hydrogen and oxygen storage equipment as well as a fuel cell. The latter will have the function of producing electrical energy and regenerating the water circulating in the loop.

  In the context of sustainable development, hydrogen can be used as fuel. Indeed, in sunny countries around the globe, there is growing interest in large-scale hydrogen production using solar energy in order to have a non-polluting, renewable energy source that can be used in the condition or as an additive to natural gas. For automotive transport, hydrogen is an interesting fuel. Indeed, the automotive industry is preparing new generations of vehicles running on hydrogen using a heat engine or a fuel cell. In both cases, the hydrogen fuel distribution infrastructure will include means for producing hydrogen. In sunny countries around the world, solar hydrogen production will provide a sustainable supply of green fuel.

  For the production of energy in air transport, the use of hydrogen is also interesting. In fact, the aeronautical industry is now examining the prospects for the propulsion of aircraft from hydrogen stored in the liquid state. The reasons for this approach are motivated by the very high mass density of hydrogen fuel energy, by the almost irreversible increase in the cost of fossil hydrocarbons and also by the desire to develop new generations of aircraft that do not produce pollution and in particular no greenhouse gases. In this perspective, Europe has studied the Cryoplane project, one of whose aims is zero pollution. This objective assumes, of course, that the hydrogen intended to feed this aircraft is produced on the ground without CO2 carbon dioxide emission.

  Various solutions for directly or indirectly producing hydrogen by dissociation of water using solar energy are known.

  For example, the indirect production of hydrogen by dissociation of water is by means of electrolysers powered by solar electricity, which can be produced by photovoltaic panels or thermodynamic conversion of solar energy. But this shift through electricity generation generally leads to expensive solutions that point to few attractive economic opportunities. Another possible route is the indirect production of hydrogen by dissociation of the water at high temperature. Such a dissociation can be done by means of thermochemical cycles, the solar radiation bringing the necessary heat to the reactions. An article by A. Steinfeld and R. Palumbo (Solar Thermochemical Process Technology Encyclopedia of Physical Science & Technology, RA Meyers Ed Academic Press, Vol 15, pp.237-256, 2001) describes a thermochemical approach to problem. They show in particular the possibility of dissociating the molecule of water by means of two-step cycles carried out in two reaction chambers: (1) in one, oxidation of a metal (Fe, Zn ...) at a temperature high (400 C) with release then evacuation of hydrogen and (2) in the other, reduction of the metal oxide at very high temperature (> 2000 C) with release and then evacuation of oxygen. The circulation of the Zn metal and the ZnO oxide from one chamber to the other makes it possible to carry out in a steady state a cycle intended to dissociate the water in order to produce hydrogen and oxygen. A disadvantage of such a thermochemical cycle is to require the implementation of two reaction chambers between which it is necessary to ensure a transfer of material as well as heat exchanges suitable for bringing the reagents to the optimum temperature. The resulting process is therefore complex and subject to irreversibilities that degrade the energy efficiency. An article by Claude Etiévant (Solar High Temperature Direct Water Splitting, Solar Energy Materials, North Holland, 24 (1991) 413-440) describes a series of studies on the direct thermal dissociation of water vapor, in one step at very high temperature (> 2000 C). To date, this process has not proved very efficient, due to insufficient separations of hydrogen and oxygen products.

  Extraction with selective permeable membranes, hydrogen and oxygen produced by dissociation of water at very high temperatures (> 2000 C) is another approach. The process described in the A. Kogan patent (U.S. Patent 5,397,559 of March 14, 1995), comprises a three-cell chamber, separated by two porous ceramic membranes performing a selective filtering of hydrogen by the Knudsen diffusion mechanism. In the first cell, subjected to the action of solar radiation, a dissociation reaction of the water vapor occurs. A hydrogen-rich mixture is transferred to the second cell while a hydrogen-rich, hydrogen-poor water vapor remains in the first cell. In the third cell, the hydrogen content of the mixture is further increased. But Knudsen diffusion membranes being very selective, the filtering of hydrogen thus produced is inefficient.

  The subject of the invention is a novel process for producing hydrogen by dissociation of water vapor by means of concentrated solar radiation, in which the thermal energy of said radiation is no longer the main driving force of this dissociation. . The invention relates to a novel process for producing hydrogen and oxygen by dissociation of water vapor by means of concentrated solar radiation. The invention relates to a new process for producing hydrogen and oxygen by dissociation of water vapor, wherein this dissociation is significantly increased by different complementary actions. The invention relates to new equipment for implementing this new process for producing hydrogen.

  To this end, it proposes a process for the production of hydrogen by a reaction of dissociation of water by means of solar radiation in a treatment chamber, this process comprising the following steps: a concentration of the solar radiation in order to to obtain a radiation intensity adapted to induce photochemical reactions of dissociation of water vapor molecules, a submission of at least one photo-catalytic target to concentrated solar radiation, a dissociation of hydrogen and oxygen molecules of vapor from boundary layer water in contact with the surfaces of said at least one target, in situ selective extraction of the hydrogen produced by means of at least one hydrogen selective extraction membrane. The method may further comprise selective in situ extraction of oxygen produced by at least one oxygen selective extraction membrane.

  Of course, the temperature of the treatment chamber will be adapted for the operation of extraction membranes.

  The surface-to-volume ratios of the at least one membrane that is selective for hydrogen and / or oxygen may be high. Indeed, it should be taken into account that the flow of hydrogen and / or produced oxygen is proportional to the surface of the membrane and that said surface is housed in a determined volume. Therefore, to obtain a flow of hydrogen and / or oxygen at least sufficient, it is interesting to have the largest possible area in the determined volume.

  Preferably, said ratios may be defined so as to obtain an ion current density of at least 1 A / cm 2.

  A direct deposition of the photo-catalytic target on said at least one selective extraction membrane with hydrogen and / or oxygen may be carried out.

  Said at least one membrane that is selective for hydrogen and / or oxygen may constitute electrolytic cells, ie cells with virtually zero electronic conduction (ae - 0). Said at least one membrane that is selective for hydrogen and / or oxygen may constitute cells with mixed conduction, ionic (6;) and electronic (6e).

  The process may include continuously introducing the appropriate pressure water vapor. The range of pressures for the dissociation reaction may be typically 3 to 15 bars and that of temperatures of 300 to 1000 C.

  The concentration rate of the solar radiation may be between 5,500 and 3,000. Advantageously, the concentration level may be not only equal to the known concentrations used but also lower, thus reducing the amount of solar energy required.

  Owing to the various characteristics of this process, it is possible, by a direct dissociation of the water vapor under the action of solar radiation, to produce hydrogen and oxygen, under conditions which are advantageous in all respects, in particular relatively moderate temperature and pressure that allow the use of common materials. This final result is obtained by the spatial conjugation of the respective results produced (1) by a concentration of solar radiation much lower than that required by the previous methods, (2) by at least one photocatalytic target. irradiated which, at a relatively moderate temperature, dissociates water vapor into its two components, and (3) by the membrane (s) associated with (a) photo-catalytic target (s) ( s), permeable to gas, which cooperate with these targets to increase the rate of dissociation of water vapor in the boundary layers.

  With regard to the intervention of the gas-permeable photocatalytic target in the physics phenomena concerned, it is necessary to compare the phenomena exploited up to now and those used in the process according to the invention. Until now, in order to dissociate a molecule of water into its components, the photonic energy of solar radiation has been converted into the most degraded form of energy, namely heat. In contrast, according to the invention, this photon energy is exploited in its noble state, so as to be immediately and mainly converted into chemical energy, resulting from the dissociation of water into hydrogen and oxygen. However, such a conversion of photon energy into chemical energy is incomplete and is accompanied by an additional conversion to thermal energy on the photocatalytic target. The thermal energy thus generated is absorbed by the photo-catalytic target and its support but the resulting infrared radiation is, in turn, directly absorbed by the water vapor molecules of the boundary layer. Which, by greenhouse effect, finally increases the temperature of this boundary layer and thus further improves the rate of conversion of photon energy into chemical energy. The overall efficiency of the process according to the invention is significantly increased and thus becomes significantly greater than the efficiency of any previous method of dissociation of water into its components, by means of solar radiation. Such a result is obtained because, contrary to what happens in many of these prior processes, the thermal energy generated by this solar radiation no longer plays the main role but only a complementary role of the photonic energy of this same radiation, directly converted into chemical energy.

  According to a variant of the process according to the invention, the dissociation rate can be increased, by ionization of the water vapor of the boundary layer, by means of a cold plasma. Thus, in the plasma production zone, the dissociation rate of the water vapor can be considerably increased, in particular by the formation of excited molecules or active ions such as H +, H2 +, OH O 02_ etc. Furthermore, the plasma will act as a source of electrons available to prime chains of dissociation reactions by the dissociative attachment process mentioned below. In order to avoid a consumption of electrical energy that is too high to create the plasma, it will be preferable to produce a cold plasma, generated, for example, by barrier or crown discharges, very short pulsed periodicals. According to another variant of the process according to the invention, the dissociation rate may be increased by the application of an electric field between the permeable membranes respectively selective for oxygen and hydrogen, when they are installed together in the reaction chamber, to better separate the negative and positive ions produced and facilitate their respective migrations to the appropriate extraction membranes.

  We will now comment further on the different steps of this process and, to do this, first present the theoretical foundations of the invention. The interest of dissociating the water in the vapor state at high temperature is dictated by considerations of thermodynamics. The total energy to be supplied is written: AH = AG + T.AS where: AH is the variation of enthalpy, AG is the Gibbs free energy variation, T.AS is the quantity of heat to be supplied, with T the temperature and AS the variation of the entropy. It will be noted that the AG energy, which is comparable to a work, can be provided in mechanical, electrical or photonic form. In the case of a contribution in the form of photon energy, the only contribution likely to intervene in this AG term corresponds to the photons directly absorbed by photo-catalytic reactions contributing to the direct dissociation of the water vapor. As for the radiation absorbed and converted into heat, it must, of course, be counted in the term T.AS and not in the term AG. Since the primary source of energy used is concentrated solar radiation, it is advantageous to use this radiation in such a way that it can be accounted for in the term AG. This implies, according to the invention, that it intervenes directly in photo-catalytic reactions of dissociation of water, rather than in the term T.AS, in the form of heat. To complete what has been said above, it may be recalled that during the dissociation of water vapor into hydrogen and oxygen, the total energy absorbed remains substantially constant, when the temperature of this vapor increases from 100 to 1000. C, however, that the Gibbs AG energy and the thermal energy T.AS, which are its components, respectively active and passive, vary almost linearly in opposite directions from each other. Under these conditions, for example between 100 and 750 ° C., the average temperature in the reaction chamber, that is to say the fraction of photon energy converted into heat, increases from 5 to 20%. The photo-catalytic water dissociation reactions occur mainly in the boundary layers in direct contact with the surfaces of the photo-catalytic targets, under the influence of the incident concentrated solar radiation. In order to eliminate the reverse reactions leading to the recombination of the dissociation products, it is necessary for at least one of these products to be extracted in situ by means of an appropriate selective membrane and, for this purpose, preferably provided with a photo-catalytic coating.

  We will now, at first, look at photo-catalytic substances. There is a wide variety and each is likely to be used to achieve said target for dissociating water vapor under the influence of solar radiation. These substances will, of course, have good chemical stability in the presence of water vapor at the high temperatures prevailing in the reaction chamber. The photo-catalytic properties of SrTiO3 strontium titanate for water dissociation were presented and discussed by F.T.Wagner, S.Ferrer, and G.A. Somorjai in Surface Science 101, p 402-474 (1980). The photo-catalytic properties of titanium dioxide TiO2 have been presented and discussed by many authors, including L.Linkous and D.Slattery in Photocatalytic hydrogen production using a dual bed photosystem, see website of the University of Central Florida.

  The photo-catalytic properties of cerium dioxide CeO2 have been presented and discussed by May-Yen Li and W.J.Chang: Department of Physics and Material Science, c.f. National Tsing Hua University, Taiwan. The main applications of these different photo-catalytic materials, which have been studied so far in the laboratories that developed them, concern (1) the manufacture of photocells and (2) the depollution of surfaces and volumes as well as that the sterilization of organic waste and various materials, by their oxidation under the action of solar radiation. One of the photo-catalytic mechanisms of water vapor dissociation results from oxidation-reduction properties produced by some semiconductors exposed to solar radiation. By way of example, such a semiconductor is titanium dioxide TiO 2 which is generally in the form of a layer of fine grains, applied by sintering on the surface of a support. Exposed to solar radiation, TiO2 titanium dioxide is the seat of an ionization mechanism resulting from the creation of electron-hole pairs that diffuse towards the surface of the grains.

  The ionization reaction of the semiconductor is written: 2 hv ù 2 e "+ 2 h + where hv denotes a photon absorbed by a photocatalytic semiconductor, e - denotes an electron passed in the conduction band and h + is a hole linked to the departure of an electron.In contact with the holes, the water vapor molecules dissociate spontaneously to form oxygen and H + hydrogen ions, according to: 2h ++ H2O '/ ZO2 + 2H +. The H + ions recombine to form H2 in contact with the electrons appearing on the surface of TiO2 grains, according to: 2 H + + 2 e + H2. The semiconductor materials other than TiO 2 presented above are also capable of carrying out the photo-catalytic dissociation reaction of the water molecules according to a mechanism similar to that described. Advantageously, it should be noted that a chain dissociation mechanism can be established when a water molecule encounters an electron on the surface of a suitable semiconductor. An example of such a mechanism is: H2O + e- - ~ H "+ OH H- + e - * H + 2 & -11- This type of reaction, called dissociative attachment of an electron is, of course very favorable for the process for producing hydrogen, according to the invention, since it makes it possible to considerably increase the dissociation rate of the water molecules, we will now be interested in the different types of membranes that are selectively permeable to water. oxygen and hydrogen, shown in Table I, attached.

  For the highly selective oxygen permeable membranes (hereinafter referred to as MPO), the material generally used is a perovskite type ceramic having high ionic conductivity at high temperature for the O 2 ions. There are two different ways to achieve membranes using this type of material: the mixed conduction membranes (MPO CM) and those operating in electrochemical pump (MPO PEC). The accompanying Figures 1a and 1b are a schematic presentation of the procedures of membranes without photocatalytic coating of Table I. Table I: Type of Material Type Mechanism of Comments Membrane Permeation MPO CM Perovskite Difference of ionic current is Fig. 1a Potential mixed conduction compensated by the current CM = ae and ai elevated chemical: electronic in the Conduction PO21> PO22 membrane volume. Mixed The motor of the transfer is the partial pressure gradient of oxygen Perovskite Pumping Two electrodes are MPO PEC Electrochemical Conduction: arranged on the other side and Fig 1b ion Difference of other high membrane, ae = 0 potential to impose a PEC = electric difference this potential. The transfer motor pump is electrochemical an electric force. Perovskite Difference of the ionic current is MPH CM Potential Mixed Potential Compensated by the Current Fig 1 c ae and i have elevated chemical: electron in the PH21> PH22 membrane volume. CM = Transfer Engine is Conduction Mixed Partial Hydrogen Pressure Gradient5 -12- MPH PEC Perovskite Pumping Two Electrodes are Fig 1d Electrochemical Conduction: Arranged from Part and PEC = Ionic Difference from Other Membrane I Pumping High , ae = 0 potential to impose an electric electrochemical difference this potential. The transfer motor is an electric force. MPH MMC Dense Metal Diffusion of Separation very selective. Fig The Pd Alloys hydrogen in The transfer engine is MMC = metals: the pressure gradient PH21> PH22 partial membrane of hydrogen. composite metal MPH DK Materials Selective diffusion diffusion. Fig 1f Microporous Knudsen The transfer motor is DK = Diffusion (metal or ceramic pressure gradient of Knudsen) partial hydrogen. In MPO CM mixed conduction membranes, the O 2 - ions diffuse from one side to the other of the membrane under the effect of the oxygen partial pressure gradient The material has a high ionic conductivity for O 2 ions. as well as a high electronic conductivity, so that an ionic current and an electronic current of opposite direction are established in the volume of the membrane, one compensating the other, the resulting electric current being zero. MPO CM membranes operating without an electric generator, it is the partial pressure gradient of oxygen and consequently the chemical potential difference which is the engine of the oxygen transfer.This operation implies that the partial pressure of oxygen upstream of the membrane is greater than that downstream: Po21> Po22.

  This assumes that the rate of dissociation of the water vapor in the boundary layer upstream of the membrane is sufficiently high. When this condition is fulfilled, the Gibbs AG free energy is entirely supplied by the fraction of solar radiation absorbed by the photochemical reactions leading to the dissociation of the water vapor. In this case there is no need for electrical energy and the radiation alone is sufficient. The MPO CM membranes can operate at high temperature, typically 800 to 1000 C. Indeed, it is now known to build such membranes, either perovskite ceramic or a metal ceramic alloy, called cermet.

  A technology of these selectively oxygen-permeable membranes is developed at the Dutch ECN Laboratory by JFVente, WGHaije and ZSRack, particularly for applications in the chemical industry or the manufacture of solid electrolytes for high-energy fuel cells. temperature of the type SOFC (acronym for Solid Oxide Fuel Cell).

  This technique is described in a data sheet, titled Mixed conducting membranes with an oxygen flux higher than 10 ml / cm2 / min, which gives a list of the many materials that can be used for their manufacture. A general presentation of the ceramic membranes concerned will also be found in the article Dense Ceramic membranes for oxygen separation in the CRC Handbook of Solid State Electrochemistry, CRC Press, New York, 1996, p.481-553. The ceramics concerned are generally oxides of La-Sr-Fe-Co, Sr-Fe-Co, La-Sr-Ga-Fe etc. These ceramics are known for their excellent selective permeability to oxygen at temperatures below 1000 C.

  According to Figure la, the permeation of oxygen is through the membrane by an ionic conduction mechanism. On the upstream face of the membrane, the oxygen molecules dissociate at the simple contact of this surface and form negative ions. These ions migrate towards the downstream face, pushed by the difference of chemical potential due to the pressure difference of oxygen and they are neutralized at their output on the downstream face, yielding four electrons that cross the membrane against the current.

  According to FIG. 1b, in membranes operating as an electrochemical pump (MPO PEC), the negative ions O 2 move from one side to the other of the membrane under the effect of an electric field produced by an electric potential difference. applied by means of electrodes, the cathode being disposed on the upstream face of the membrane and the anode on the downstream face. For this type of operation, a material having a high ionic conductivity for O 2 - ions and a zero electronic conductivity is used, this operating mode is similar to a high temperature electrolysis mechanism. is very similar to that of a SOFC cell and the material will for example be yttrium-doped zirconia An advantage of this mode of operation is to allow the passage of oxygen through a membrane even when the partial pressure upstream is much lower than downstream: P021 <P022.

  Permeable membranes highly selective for hydrogen (hereinafter referred to as MPH), are of several types: (1) mixed conduction perovskite ceramic membranes (MPH CM), (2) perovskite ceramic membranes operating in electrochemical pump (MPH) PEC), (3) dense composite metal membranes (MMC) and (4) Knudsen diffusion membranes (MPH DK). The procedures of these first two types of membranes are shown in Figures 1c and Id attached.

  According to FIG. 1c, in the mixed-conduction perovskite ceramic membranes MPH CM, the material used has, at elevated temperature, a high ionic conductivity for H + ions as well as a high electronic conductivity. According to this figure, the positive ions H + diffuse from one side to the other of the membrane under the effect of the hydrogen partial pressure gradient. A ionic current and an electronic current of opposite signs are established in the volume of the membrane, the resulting electric current is then zero. MPH CM membranes therefore operate without an electrical generator, the transfer motor is the partial pressure gradient and therefore the chemical potential difference of hydrogen between the upstream and downstream of the membrane. This operation implies that the hydrogen partial pressure upstream of the membrane is greater than that downstream: PH21> PH22. In the perovskite ceramic membranes selectively permeable to hydrogen, the material constituting this type of membrane is a ceramic having at high temperature electronic and protonic mixed conduction properties. The principle of such membranes differs from that described above for the oxygen extraction membranes in that the ionic conduction of the O 2-ions has been replaced by that of the H protons. Complex materials making it possible to produce such membranes are presented. in the article by EA Payzant et al., Pyrochlore-Perovskite Protons Transfer Membranes, published in Oak Ridge's FY 2004 Progress Report.

  According to FIG. 1d, in membranes operating as an electrochemical pump (MPH PEC), the H + ions move from one side to the other of the membrane under the effect of an electric field produced by an applied electric potential difference. by means of electrodes, the anode being disposed on the upstream face of the membrane and the cathode on the downstream face. For this type of operation is used a particular material having a high ionic conductivity for H + ions and zero electronic conductivity. Such material is described in an article by Hiroyasu Iwahara entitled Hydrogen Pumps Using Proton-Conducting Ceramics published by Elsevier in Solid State Electronics 125 (1999), pages 271-278. The mode of operation of these membranes can be likened to a high temperature electrolysis mechanism. An advantage of this mode of operation is to allow the passage of hydrogen through a membrane even when the upstream partial pressure much lower than the downstream: PH21 <PH22.

  According to Figure 1c, in dense metal membranes, based on the mechanism of diffusion of hydrogen in metals, palladium or its alloys are generally used. The composite metal membranes will hereinafter be designated by the acronym MPH MMC. These membranes are described in the Patent Composite Structures of Selectively Hydrogen Permeable Membranes and Gas Processors by Making Use of WO 02/0661 A2, filed by CETH, the proprietor of the present application. The operation of this type of membrane implies that the temperature is between 300 and 650 ° C. and that the partial hydrogen pressure upstream of the membrane is greater than that downstream: PH21> PH22.

  According to Figure 1f, in micro-porous membranes based on the Knudsen diffusion mechanism (MPH DK), very light hydrogen diffuses faster than heavier gases, allowing for selection. The microporous material is generally a ceramic, in particular alumina. These membranes generally have poor selectivity. The principle of operation of these membranes implies that the hydrogen partial pressure upstream of the membrane is greater than that downstream: PH21> PH22. Porous ceramic membranes operating according to the Knudsen diffusion principle are commercially available. They have certain advantages: good stability at high temperature, high value of their gas permeability and low cost.

  Nevertheless, they are very selective.

  Hydrogen production equipment for carrying out the process according to the invention comprises: a convergent optical device, adapted to best concentrate the incident solar radiation in its focus; an apparatus (12) for dissociating the water vapor placed at the focus of said optical device and comprising an enclosure closed by a concentrated solar radiation access window, said chamber comprising a treatment chamber in which the focal zone is formed; irradiated from the optical device; said treatment chamber comprising: a steam inlet, the water being heated and vaporized by means of at least one internal heat exchanger and / or external to said treatment chamber, at least one photo-catalytic target installed in said said irradiated focal zone, at least one selectively hydrogen-permeable membrane disposed in the vicinity of said at least one photo-catalytic target and externally connected to extract hydrogen. As regards the necessary optical concentration, it will be between 500 and 3000. It is defined by the ratio between the intensity of the light radiation incident on the optical system and the intensity of the concentrated light radiation in the focal spot. . Many convergent optical devices are known and commercially available. Parabolic mirrors of revolution associated with automatic piloting devices are industrially produced to concentrate the solar radiation on a high temperature receiver intended to operate a Stirling engine or a gas turbine, in order to produce electricity using solar energy. A review article describing such systems, entitled Dish-Stirling Systems: An Overview of Development and Status, was published by Thomas Mancini et al. in Journal of Solar Energy Engineering May 2003, 8 vol.125 pp135 -151. According to other embodiments that are also possible, the convergent optical device or optical concentrator may be an orientable Fresnel mirror, consisting of reflective facets, or a heliostat field, similar to that of the Themis solar power plant built in France, or still a double reflection system, consisting of a fixed parabola and adjustable heliostats, realized in France by the CNRS for the solar ovens of Mont-Louis and Odeillo.

  Said treatment chamber may further comprise at least one selectively oxygen-permeable membrane connected to the outside.

  The membranes selectively permeable to oxygen and / or hydrogen installed in the treatment chamber may be in the form of gloves fingers and / or radial strips and / or honeycombs. When said at least one membrane that is selectively permeable to oxygen and / or hydrogen, installed in the treatment chamber, is of the electrolytic type, the anode of each membrane is connected to the cathode of the contiguous membrane. The equipment may further comprise at least one of the following elements: for each membrane that is selectively permeable to hydrogen and / or oxygen, a heat exchanger whose input of the hollow active element is connected to said membrane selectively permeable to hydrogen and / or oxygen, the outlet of this hollow element opening respectively to a device for exploiting hydrogen and / or oxygen, - a water tank and a suitable pump supplying water to the casings of the active hollow element (s) and a tubing connecting the outlet (s) of this element (s) to the treatment chamber for feed it with water vapor.

  According to one variant, a hydrogen production equipment for carrying out the method according to the invention may comprise: a convergent optical device, adapted to best focus the incident solar radiation in its focus; a device for dissociating the water vapor placed at the focus of this optical device and comprising an enclosure closed by an access window for concentrated solar radiation, said enclosure comprising a treatment chamber comprising a reaction chamber in which is formed the irradiated focal zone of the optical device and an extraction chamber communicating with the reaction chamber through the hollow active element of a heat exchanger: said reaction chamber comprising: an inlet of water vapor formed after passing through the envelope of the heat exchanger, at least one photocatalytic target installed in said irradiated focal zone, at least one membrane that is selectively permeable to one of the two gases, oxygen and hydrogen, disposed in the close vicinity of said at least one less a photo-catalytic target and connected to the outside. Said extraction chamber comprising at least one membrane selectively permeable to one of the two gases, hydrogen and oxygen, connected to the outside.

  According to this variant, said equipment may further comprise at least one of the following elements: a water tank and a suitable pump, between the pump and the heat exchanger, at least two heat exchangers, the hollow active elements of these at least two exchangers being arranged in series and respectively supplied by the hydrogen and oxygen streams leaving the extraction and reaction chambers and the envelopes of these active elements being supplied with water for example by said reservoir and said pump, a conduit of evacuation of the residual steam from the extraction chamber connecting the outlet of this chamber to a suction pump, to the other inlet of which terminates the outlet of the shell of the heat exchanger, the outlet of this horn being connected by tubing to the steam inlet mouth of the reaction chamber.

  Various examples of architectures of the reaction chambers for the dissociation of water into its components according to the invention will now be presented. These are briefly shown in Table II, attached. In this table, there are seven configurations characterized in that the dissociation apparatus may comprise either a reaction chamber and an extraction chamber (4 cases) or a reaction chamber only (3 cases). Table II: Reaction Chamber Extraction Chamber Energy Membrane Agent (s) Diaphragm penetrating into installed dissociation (s) installed H2O reactor Radiation Target Photo- MPO CM MPH solar catalytic Recovered gas: 02 Extracted gas: H2 concentrated Retentate: H2O + H2 Retentate: H2O Radiation Target Photo- MPO PEC MPH solar catalytic Gases Extracted gas: 02 Extracted gas: H2 concentrated Retentate: H2O + H2 Retentate: H2O Radiation Target Photo- MPH CM MPO solar catalytic Extracted gas: H2! Extracted gas: 02 concentrated Retentate: H2O + 02 Retentate: H2O Radiation Target Photo- MPH PEC MPO solar catalytic Gas extracted: H2 'Gas extracted: 02 concentrated Retentate: H2O + 02 Retentate: H2O Radiation Target Photo- MPO CM No solar second catalytic + concentrated chamber MPH Gases extracted: H2 and 02 Retentate: H2O Radiation Target Photo- MPO PEC No second solar catalytic + concentrated chamber MPH Gases extracted: H2 and 02 Retentate: H2O Radiation Target Photo- MPO PEC + MPH PEC Gases extracted: H2 and 02 Retentate: H2O In all cases, the reaction chamber is exposed to concentrated solar radiation. This chamber contains water vapor delivered by a suitable feeder. In the reaction chamber, the concentrated solar radiation penetrates through an access window such as a transparent window, for example made of quartz, and strikes at least one photocatalytic target deposited in the vicinity of at least one membrane. 'extraction. This extracts, depending on the case, the oxygen or hydrogen produced in contact with the surfaces of said at least one photocatalytic target. Upstream of said at least one membrane remains a residue (retentate) composed of gases other than the extracted gas. In the cases where the dissociation apparatus comprises a reaction chamber and an extraction chamber, this residue (retentate), composed of water vapor mixed with hydrogen or oxygen is then conveyed to the second chamber in which is disposed at least one membrane which extracts hydrogen or oxygen therefrom. The residue (retentate) of this second selection operation is mainly steam which is recycled by mixing it with the steam injected into the reaction chamber. In Table II, there are also three configurations characterized by the fact that the apparatus comprises only the reaction chamber. In each of these three configurations, the hydrogen and oxygen extraction membranes, if any, are arranged side by side in this chamber. It should be noted that in this case, the circulation of the residue is simplified: the water vapor remains in the reaction chamber, the water vapor provided just compensating for the dissociated fraction and converted into oxygen and hydrogen.

  These different configurations will now be commented with reference to the illustrations of Figures 2a, 2b, 2c and 2d.

  According to FIG. 2a, in the case of a mode of operation of the method based on the sole use of mixed electron and ionic conduction membranes, no electrical energy is necessary. The Gibbs AG free energy term is entirely provided by the photon energy hv not converted into heat from solar radiation, absorbed by the photocatalytic target, to dissociate the water vapor to hydrogen and oxygen. This dissociation of the water, in contact with the photo-catalytic target under the effect of solar radiation, generates a thin layer of gas rich in dissociation products, in particular H2 and O2. When the partial pressure of the oxygen created in the gas-permeable photocatalytic layer, in the vicinity of a MPO CM mixed conduction membrane, reaches a value greater than the oxygen partial pressure downstream, then a transfer of oxygen through said membrane is established. According to FIG. 2c, an identical mechanism occurs when the partial pressure of the hydrogen produced in the photocatalytic layer in the vicinity of a mixed conduction membrane MPH CM reaches a value greater than the partial pressure of hydrogen. downstream. It then establishes a transfer of hydrogen through said membrane.

  According to FIGS. 2b and 2d, two types of membranes operate according to an electrochemical pumping mode and the process for producing hydrogen according to the invention is in both cases a method of electrolysis of high temperature water vapor. . The electric field required for this purpose is maintained by an electric potential difference applied between the two electrodes of the membrane. This implies that an electric generator supplies the energy consumed by the membrane concerned. The electrical energy consumed will be even lower than the difference in the partial pressures of the pumped gas will be lower between the upstream and downstream of the membrane. This shows the advantage of having upstream of the membrane a photo-catalytic layer which, under the effect of solar radiation, locally increase the partial pressure of the dissociation products. The electrochemical reactions involved in high temperature electrolysis are quite complex, but can be written in a simplified way. According to FIG. 2b, in the case of an MPO PEC electrolysis cell using a solid electrolyte permeable to oxygen ions, the reactions are as follows: Cathodic side reaction (-): 2H 2 O + 4th --- 202 + 2 H2 - Anodic side reaction (+): 202 "ù-02 + 4e- The water vapor arrives on the cathodic face in contact with which the dissociation reaction occurs. The residue (retentate) is formed by the undissociated water vapor which leaves again enriched in hydrogen. On the anodic side, the O 2 ions that have passed through the electrolyte membrane recombine to form oxygen. The released electrons are collected by the anode and are fed back to the cathode via the external electrical circuit into which an electric generator is inserted. According to FIG. 2d, in the case of an electrolysis cell using a solid electrolyte conducting hydrogen ions H + (MPH PEC), the dissociation of the water takes place on the anodic face and the reactions are as follows: - Reaction anodic side (+): 2 H2Où> 4 H ++ O2 + 4th - Reaction cathodic side (-): 4 H ++ 4th "ù 2 H2 The water vapor arrives on the anodic face on which it is partially dissociated and released enriched with oxygen (= retentate) On the cathodic face the ions H + which crossed the electrolytic membrane form hydrogen by recombining with the electrons driven by the external electric circuit .In both cases of electrolysis with high temperature described above, the potential difference to be applied between the electrodes is directly related to the difference in chemical potential on either side of the electrolytic membrane In a simple electrolysis operation, in the absence of solar radiation,the necessary electrical energy will be the AG of the reaction. In the presence of solar radiation, the necessary electrical energy will be reduced by the photon energy that has contributed to the photochemical dissociation of the water vapor. Thus it will be possible to significantly reduce the electrical energy consumed by the process of electrolysis of water vapor at high temperature. At any suitable temperature, this mechanism of dissociation of the water vapor, in a boundary layer in contact with the grains of an illuminated photocatalytic target and in the vicinity of a membrane that is selective for one of the gases produced, is very different and much more efficient than the decomposition of water vapor in a homogeneous medium, in thermodynamic equilibrium. For the process to be effective, it is essential that at least one of the gases produced be extracted from the reaction medium in a shorter time than its recombination time with the other gas produced. To achieve this objective, it is necessary that the dissociation reaction take place in the vicinity of the extraction membrane and, therefore, the photo-catalytic target will preferably be directly deposited on one of the membranes concerned or, if necessary, on its upstream electrode.

  Said at least one membrane located in the first chamber, when it is directly exposed to solar radiation, will have a high active surface radiation. For this reason it should have forms allowing a high surface area ratio and have, for example, tubular, lamellar or alveolar shapes.

  In the case of electrochemical pumping membranes used for high temperature electrolysis, the membrane is a solid electrolyte and has a high ionic conductivity in the considered temperature range and an almost zero electronic conductivity, as shown in FIGS. 1b. , Id, 2b and 2d. The electrodes are disposed on both sides of the solid electrolyte membrane. They must also allow the flow of gases produced in contact with the solid electrolyte membrane. For this reason, they necessarily consist of porous materials. The materials used to form the electrodes of a high temperature electrolysis cell are known because of the work currently being carried out worldwide to develop high temperature electrolyzers on the one hand as well as SOFC type fuel cells. 'somewhere else.

  Advantageously, in the case of a high temperature electrolysis, the direct absorption of the concentrated radiation, by photochemical reagents which participate in the dissociation of the water, reduces the potential difference between the electrodes of the electrolysis cell and hence the electrical energy consumed. But with a small potential difference applied in parallel on all the membranes, the generated electric current can be relatively high. Since the current density per unit area must remain compatible with the limit imposed by the material used, it may be necessary to arrange the membranes in series and to increase the potential difference applied to the whole, to decrease consequently the amplitude of the current generated.

  In the case where the membranes used are mixed conduction membranes, that is to say having a conductivity that is both ionic and electronic, the reactions on either side of the membrane are the same as those indicated above. high. Nevertheless, in this case, the transfer of oxygen or hydrogen through the membrane can only take place if the partial pressure of this gas is stronger upstream than downstream. This assumes that the rate of dissociation of the water vapor in the boundary layer upstream of the membrane is sufficiently high. When this condition is fulfilled, the Gibbs AG free energy is entirely supplied by the fraction of solar radiation absorbed by the photochemical reactions leading to the dissociation of the water vapor. In this case there is no need for electrical energy and the concentrated solar radiation alone is sufficient. On the other hand, the extraction of one or both of the dissociation products constantly moves the thermodynamic equilibrium, so that the reaction of decomposition of the water is carried out at a lower temperature than in the case of a thermodynamic equilibrium reaction without product extraction. Whereas in the mechanism of dissociation of water vapor at thermodynamic equilibrium in a homogeneous medium, it is necessary to reach a temperature well above 2000 C to observe an appreciable dissociation rate, in the case of a dissociation on the grains of water. a photo-catalytic target illuminated by solar radiation, accompanied by an immediate extraction of the gases produced, the dissociation takes appreciable values at much lower temperatures, of the order of 300 to 1000 C. What can obtain with a concentration of solar radiation much lower than what was necessary until now. The features and advantages of the invention will appear more precisely following the following description of two embodiments of the invention given by way of non-limiting examples, with reference to the accompanying drawings. in which: Figures 1 and 2 are illustrations of the selective membranes discussed above;

  Figure 3 is a schematic representation of a hydrogen generating equipment using solar energy according to the invention;

  Fig. 4 is a schematic representation of a two-chamber steam dissociation apparatus embodying the method of the invention;

  Figure 5 shows a schematic section of two electrolytic membranes in the form of glove fingers.

  According to FIG. 3, a hydrogen production equipment according to the invention comprises a mirror 10, in the form of a parabola of revolution, mounted on a foot 11 and orientable towards the sun, and a device for dissociating water vapor. 12, disposed at the focus of said mirror and carried by an arm 13 secured to this mirror. By way of example, the opening surface 25 of the mirror 10 may be 50 m2, the focal area 5 dm2 and the volume of the apparatus 12, 20 dm3 approximately.

  According to Fig. 4, the water vapor dissociation apparatus 12 is a bell-shaped, thick-walled metal enclosure 30 defining a process chamber, which encloses a reaction chamber 16 disposed at the front (Lower part) and an extraction chamber 18, installed behind (upper part). Suitable metal means a metal which is chemically stable with respect to the reactants and products at the temperature used, such as a steel. stainless. The reaction chamber 16 is closed by a concave quartz port 20, mounted fixed and sealed between annular jaws 21, integral with the wall of the bell 14. This porthole is transparent to solar radiation, stable at high temperature and able to withstand the high internal pressure of the chamber 16. In addition, this bell 14 is insulated by a thick layer 22 of thermal insulation, silica wool for example.

  In the irradiated focal zone, generated by the mirror 10 in the reaction chamber 16, is installed a first group 24 of membranes 26 selectively permeable to oxygen, one of the MPO CM or MPO PEC types defined above. These membranes 26 may have the form represented by rigid glove fingers and they are each coated with a layer, for example sintered, of a photocatalytic semiconductor, such as titanium dioxide TiO 2. The group 24 comprises a large number (from ten to twenty, for example) of membranes 26, regularly arranged and sealingly attached to the bottom 28 of a round collector housing 30, made of a suitable material (ceramic or metal) connected to a duct 32 for evacuating the product oxygen. The diameter of the window 20 is substantially equal to the diameter of the circle in which the group 24 of the membranes 26 is installed. If the membranes 26 are of the MPO PEC type, the internal and external electrodes of two adjacent membranes will be connected in series by means of, for example, For example, insulated bushings installed in the metal base 28 of the collector housing 30. Instead of a form of glove fingers, the membranes 26 may be in the form of radial slats or any other shape assuring them a surface-to-volume ratio. as high as possible. The bottom 34 of the reaction chamber 16 is pierced by an opening 35, on which is connected the inlet mouth of a hollow active element 36 and high surface area ratio, (the serpentine shown, for example) , belonging to a heat exchanger 38, for confined fluids, comprising a casing 40 enclosing this element 36. The outlet mouth 37 of this element 36 leads to the inlet of the extraction chamber 18. demineralised water 42, provided with a feed pump 44, at high pressure and variable flow, and a distribution duct 46, is disposed in the vicinity of the apparatus 12. Two heat exchangers 48 and 50, connected to the exchanger 38, are installed along the conduit 46, outside the device 12. The envelopes 52 and 54 of the exchangers 4850 are arranged one after the other, in series with the conduit 46 and the output of the envelope 54 is connected to the entrance of the envelope 40 of the exchange The hollow active elements 56 and 58 internal to the envelopes 52 and 54 are respectively arranged in series with the hydrogen extractor 60 and oxygen extracting pipes 32. The various fluids concerned flow counter-current. in the active hollow elements and in the envelopes of these heat exchangers. The inlet of the casing 40 of the exchanger 38 is connected by a duct 39 to the outlet of the casing 54 and the outlet of the casing 40 is connected by a duct 62 to the first inlet of a trunk 64 The second inlet of this horn 64 is connected by a duct 66 to the outlet of the extraction chamber 18 and its outlet, connected by a pipe 68 to the inlet of the reaction chamber 16.

  The active elements 36, 56 and 58 of the heat exchangers 38, 48 and 50 will preferably have a very high thermal conduction, and for this purpose they may be made in the form of a stack of hollow, metal fins. thin, thin-walled, stamped and welded, these fins being connected to two transverse collectors and separated by narrow inter-fin spaces. Such heat exchange active elements are described in the European patent application of the Japanese company EBARA, published under the number EP 1 122 505 in August 2001.

  The extraction chamber 18 contains a stack 70 of highly hydro-selective pallet-tight membranes 72 of one of the MPH CM, MPH PEC or MPH MMC types defined above. In accordance with the international patent application WO 02/0661 A2, each puck may comprise a thin metal layer deposited on a porous disk-shaped substrate encased in a sealing ring. Such a membrane 72 comprises a fine gas extraction pipe 74 pierced in said ring and joining the substrate. These crowns are approximately 10 cm in internal diameter and 5 mm in thickness. The membranes 72 are stacked with spacer pads 76, in the form of rings, thus creating separation spaces of the same thickness. The number of membranes 72 in the stack 70 is from ten to twenty and their extraction ducts 74 are connected to a common manifold 78, ensuring the evacuation of the hydrogen produced, connected to the outlet pipe 60 of the chamber 18. If the membranes 72 of the extraction chamber 18, are of the MPH PEC type, they will preferably keep their shape in pallets and remain superimposed in the stack 70. In this case, the anode and the cathode two contiguous membranes will be directly connected to each other.

  For the realization of these MPO PEC membranes, perovskite ceramics conducting 02 "ions will be used.In the Proceedings of the Gaitherburg Workshop (4-5 March 2004), S. Herring describes the composition of such a cell. high-temperature electrolysis The dense electrolyte membrane consists of yttrium-stabilized zirconia The cathode is a porous layer of a manganese-lanthanum-doped strontium mixture and the anode is a porous manganese-lanthanum sintered layer. The combination of these layers constitutes a sandwich structure which can be easily transposed to the finger-tip geometries, shown in FIG. 5 described below, of the membranes that can be used in the reaction chambers and the extraction chambers. the apparatus for dissociating water vapor, which is the subject of the invention, is the same for the shuffle membranes of the extraction chamber 18. Membranes 26 and 72 respectively permeable to oxygen and hydrogen can be reversed, the membranes installed in the reaction chamber 16-keeping their shape in thimbles and those installed in the extraction chamber 18, their form in palets. In the case where the hydrogen-permeable membranes installed in the reaction chamber are of one of the MPH CM or MPH PEC types, these membranes will again have the form of glove fingers or radial lamellae. On the other hand, membranes of the MPH MMC type can not be installed in the reaction chamber 16 since their maximum operating temperature can not exceed 650 ° C. and the temperature in this chamber is approximately 750 ° C. In conclusion, it should be noted that the temperature in the extraction chamber 18 before a priori be significantly lower than that of the reaction chamber, the oxygen-permeable membranes will preferably be installed in the reaction chamber, since their optimum operating temperature is 800 to 900 C. As an alternative to the water vapor dissociation apparatus 12, it will comprise only the reaction chamber 16, the extraction chamber 18, the heat exchanger 38 and the trunk 64 being removed . In this case, the membranes installed in the reaction chamber will be of two kinds, one permeable to oxygen and the other permeable to hydrogen, but all will have a high surface-to-volume ratio and will be adapted to function properly at the same time. relatively high temperatures (800 to 900 C). These two kinds of membranes will be associated with two kinds of individual collectors, respectively assigned to oxygen and hydrogen and connected to two common collectors. The membranes of the first group will, for example, be gathered in the center of the collector box and those of the second group installed around it. The collector box will comprise two cells provided with extraction ducts, one round in the center and the other in a ring around it. The other subsets of the water vapor dissociation apparatus 12, described in FIG. 4, remain unchanged.

  According to FIG. 5, two contiguous MPO PEC membranes are shown in section, in a possible embodiment, different from what is said above. According to this FIG. 5, the glove finger membranes 80 and 81 each comprise a tubular layer 82 and 83 constituted by the solid electrolyte. These two layers are integral with a plane support 84 constituting a wall of the collector housing 30 and the whole forms a gas-permeable sintered assembly made of perovskite ceramics of ion-conductive type. The inner and outer walls of the two substrates 82 and 83 are coated with gas-permeable sintered electrodes, namely an anode 86-87 on their inner faces and a cathode 88-89 on their outer faces. The anode 86 is connected to the cathode 89 by a conductor 90 which passes through the support 84. On the cathodes 88-89 are deposited gas-permeable photocatalytic layers 92-93 and an identical layer 94 on the exposed parts of the With these various arrangements, the operation of equipment 12 for dissociating water vapor using solar energy is clearly apparent. The solar radiation, concentrated by the parabolic mirror of revolution 10, enters the reaction chamber 12 through the concave porthole 20 and illuminates all the photocatalytic coatings of the glove finger membranes 26. On this occasion, a large part of the photon energy of this concentrated radiation is directly converted into chemical energy, the latter resulting from the dissociation of oxygen and hydrogen from the water vapor present in the reaction chamber. In addition, because of the conversion of the remaining portion of this photon energy into thermal energy absorbed by the photo-catalytic coating and its support, the re-emitted infrared radiation increases the temperature of the dissociated water vapor boundary layer in contact with this coating and, thereby, improves the dissociation rate. The water vapor present in the reaction chamber 16 is there with a pressure of 5 to 15 bar and a temperature of up to 800 C in said boundary layer. The means to obtain this state of affairs will be presented and commented on later. As the dissociation of water vapor occurs, under the action of concentrated solar radiation, the partial pressure of oxygen increases in said boundary layer and, when this pressure exceeds a certain threshold this gas selectively passes through the membranes 26 to be collected in the collector housing 30. The oxygen extraction conduit 32 causes this gas, thus filtered and collected, to penetrate into the hollow active element 58 of the exchanger Thermal 50. This stream of oxygen is cooled and then comes out for proper storage. As the oxygen produced leaves the reaction chamber 16, through the membranes 26, the very hot mixture (800 C) remaining, composed of undissociated water vapor and hydrogen, in turn leaves this chamber 16, through the opening 35 formed in the bottom of this chamber. This mixture then enters the hollow active element 36 of the heat exchanger 38 and it cools to get out at about 450 C and enter the extraction chamber 18. In this chamber 18, the hydrogen present is filtered by the membranes 72 in the form of pallets stacked with the appropriate separation spaces 76, then collected by the manifold 78 and discharged through the extraction duct 60. This duct 60 causes the stream of hydrogen thus produced to enter the room. hollow active element 56 of the heat exchanger 48. This stream is cooled there and then comes out for proper storage. Under the action of the pump 44 of high pressure feed and adjustable flow, a stream of demineralized water, at the ambient temperature of the tank 42, is injected in the liquid state into the casing 52 of the first heat exchanger 48 equipment. Due to the high temperature (400 C) of the hydrogen which circulates in the hollow active element 56, the liquid water which penetrates, against the current of the preceding, into the envelope of this element is vaporized and the steam produced at a temperature of about 300 ° C. Under the action of the pump 44, this water vapor passes through the casing 54 of the heat exchanger 50, the hollow active element 58 of which is traversed by the flow of water. very hot oxygen (800 C) leaving the collector housing 38. The stream of water vapor, which comes out at a temperature of about 500 C, enters the envelope 40 of the third heat exchanger 38 of the equipment. In this exchanger, the temperature of this stream of water vapor increases under the action of the hot steam and very hot hydrogen mixture (800 C) which flows through the hollow active element 36. The stream of very hot water vapor , which leaves via the conduit 62, penetrates into the tube 64 to which leads the conduit 66 which ensures the evacuation of water vapor at approximately 400 C in the extraction chamber 18 after filtering and evacuation hydrogen. The resulting stream of water vapor is injected into the reaction chamber 16, where there is a high pressure, under the action of the pump 44, and a high temperature under the action of concentrated solar radiation which enters through the porthole 20.

  According to the method of dissociation of the water vapor defined above, it is expected to improve the rate of this dissociation by creating a cold plasma near the selective membranes coated with a photocatalytic layer. According to the definition used by the specialists, a plasma is said to be cold when the ionic and molecular temperatures in the ionized zone remain very cold compared to that of the electronic population. As a result, electronic collisions are able to create active radicals that participate in the chemical reactions of dissociation of water vapor. In such plasmas, the cooling of the electrons takes place partly by conversion to heat and partly by the chemical reactions thus produced. In this way, it is possible to selectively increase the latter with a small amount of electrical energy. Given the very short collisions time, it is possible to create cold plasmas in a gas at high pressure for very short pulses, less than the time of establishment of a thermal plasma (hot plasma).

  Different types of pulsed discharges make it possible to create a cold plasma having the desired properties, for example pulsed periodic crown discharges, or pulsed barrier discharges. Thus, for example, to achieve a pulsed barrier discharge can be used a composite device in the form of pencil. On the axis of the rod is disposed a filiform metal electrode. The latter is coated with an insulating ceramic sheath. This device is inserted into the water vapor and the central electrode is connected to a generator of very short high voltage pulses (pulse duration one to a few microseconds, one millisecond recurrence time, pulse voltage 15 kilovolts) . A sheath of ionized gas rich in chemically active radicals is then formed around the pencil and is involved in the dissociation process of the water. It is advantageous to regularly distribute such devices in the gaps between the thimbles of the membrane device 24.

  In addition, it will be recalled that the temperature of the appropriate metal wall of the bell, which surrounds the reaction chamber in which the solar radiation is focused, must be maintained at a value lower than the ceiling authorized for this metal. To do this, a solution will consist in applying, on the internal face of this wall, a coil with contiguous turns, made of a metal having a temperature ceiling much higher than the previous one, and to circulate in this coil the vapor stream of water just before being injected into this reaction chamber.

  The invention is of course not limited to the embodiments illustrated and / or described above. In this respect, it will be noted for example that the steerable parabolic mirror 10 may be replaced by any other form of optical concentrator, in particular those indicated above: a set of reflecting facets constituting a steerable Fresnel mirror, a heliostatic field or a double system including a fixed parabolic mirror and adjustable flat mirrors. With regard to the reversal of the locations of membranes selectively permeable to oxygen or to hydrogen, in the reaction and extraction chambers, it should be noted that the thing is entirely possible, according to the invention, but not necessarily equivalent to the provisions of the apparatus 12 shown in Figure 4. Indeed, the temperatures in these two chambers being very different from each other (about 800 and 400 C) and perovskite ceramic membranes used, Suitable for optimum operating temperatures, this inverted arrangement can not function as well as the previous one. This, all the more so, that the membranes selectively permeable to hydrogen with a palladium layer, represented in FIG. 4, have an optimum operating temperature of about 450.degree.

Claims (21)

  A process for the production of hydrogen by a dissociation reaction of water by means of solar radiation in a treatment chamber, characterized in that it comprises: a concentration of solar radiation in order to obtain an intensity of radiation adapted to induce photochemical reactions of dissociation of water vapor molecules, a submission of at least one photocatalytic target to concentrated solar radiation, a dissociation in hydrogen and oxygen of water vapor molecules of a boundary layer in contact with the surfaces of said at least one target, in situ selective extraction of hydrogen produced by means of at least one hydrogen selective extraction membrane.   2. Process according to claim 1, characterized in that it comprises an in situ selective extraction of the oxygen produced by means of at least one selective oxygen extraction membrane.   3. Method according to one of claims 1 or 2, characterized in that the surface-to-volume ratios of said at least one membrane selective for hydrogen and / or oxygen are defined so as to obtain a current density. ionic strength of at least 1A / cm2.   4. Process according to claim 1, characterized in that the pressure range for the dissociation reaction is typically 3 to 15 bar and that of the temperatures of 300 to 1000 C.   5. Method according to claim 1, characterized in that the solar radiation concentration ratio is between 500 and 3000.   6. Method according to claim 1, characterized in that the rate of dissociation of the water vapor molecules of the boundary layer is increased, by ionization of these molecules, by means of a cold plasma, generated in particular by barrier discharges. or crown, very short pulsed periodicals.   7. Method according to claim 2, characterized in that the rate of dissociation of the water vapor molecules of the boundary layer is increased, by the application of an electric field between the permeable membranes respectively selective for oxygen and hydrogen.   8. Hydrogen production equipment for implementing the method according to claim 1, characterized in that it comprises: - a convergent optical device (10), adapted to best focus in its focus incident solar radiation; an apparatus (12) for dissociating the water vapor placed at the focus of said optical device and comprising an enclosure closed by a concentrated solar radiation access window, said chamber comprising a treatment chamber in which the focal zone is formed; irradiated from the optical device; said treatment chamber comprising: a steam inlet, the water being heated and vaporized by means of at least one internal heat exchanger and / or external to said treatment chamber, at least one photo-catalytic target installed in said said irradiated focal zone, at least one membrane selectively permeable to hydrogen, disposed in the vicinity of said at least one photo-catalytic target and connected externally to extract the hydrogen.   9. Equipment according to claim 8, characterized in that said treatment chamber further comprises at least one membrane selectively permeable to oxygen.   10. Equipment according to claim 8, characterized in that it further comprises at least one of the following elements: - for each membrane selectively permeable to hydrogen and / or oxygen, a heat exchanger (48, 50) whose inlet of the hollow active element (56, 58) is connected to said membrane selectively permeable to hydrogen and / or oxygen (26, 72), the outlet of this hollow element opening respectively to a device for operating hydrogen and / or oxygen, - a water tank (42) and a suitable pump (44) supplying water to the envelopes (52, 54) of the element (s) hollow asset (s) (56, 58) and a tubing (68) connecting the outlet (s) of this element (s) to the treatment chamber (16) for supplying steam thereto; water.   11. Equipment for producing hydrogen for the implementation of the method according to claim 1, characterized in that it comprises: - a convergent optical device (10), adapted to best focus in its focus incident solar radiation; an apparatus (12) for dissociating the water vapor placed at the focus of said optical device and comprising an enclosure closed by a concentrated solar radiation access window, said enclosure comprising a treatment chamber comprising a reaction chamber in which the irradiated focal zone of the optical device is formed and an extraction chamber communicating with the reaction chamber through the hollow active element of a heat exchanger: said reaction chamber comprising: an inlet of water vapor formed after passing through the envelope of the heat exchanger, at least one photocatalytic target installed in said irradiated focal zone, at least one membrane selectively permeable to one of the two gases, oxygen and hydrogen, disposed at close vicinity of said at least one photo-catalytic target and connected to the outside. said extraction chamber comprising at least one membrane selectively permeable to one of the two gases, hydrogen and oxygen, connected to the outside.   12. Equipment according to claim 11, characterized in that it further comprises at least one of the following elements: a water tank (42) and a suitable pump (44), between the pump (44) and the exchanger thermal (38), at least two heat exchangers (48, 50), the hollow active elements (56, 58) of these at least two heat exchangers being arranged in series and respectively fed by the currents of hydrogen and oxygen leaving the extraction chambers (18) and reaction chambers (16) and the envelopes (52, 54) of these active elements being fed with water for example by said reservoir and said pump, a conduit for evacuation of the residual steam of the extraction chamber connecting the outlet of this chamber to a suction horn, to the other inlet of which terminates the outlet of the shell of the heat exchanger, the output of this horn being connected by a tubing to the steam inlet mouth of the reaction chamber.   13. Equipment according to at least one of claims 8, 9 or 11, characterized in that said at least one photocatalytic target is directly deposited on said at least one membrane selectively permeable to hydrogen and / or oxygen. - 39 -   14. Equipment according to at least one of claims 8 or 11, characterized in that said at least one photocatalytic target comprises titanium dioxide TiO2 or strontium titanate SrTiO3 or cerium dioxide CeO2.   15. Equipment according to one of claims 8 or 11, characterized in that the window (20) closing the reaction chamber (16) is transparent to solar radiation, stable at high temperature and adapted to withstand high pressure. 10   16. Equipment according to at least one of claims 8, 9 or 11, characterized in that said at least one membrane selectively permeable to hydrogen and / or oxygen is made of perovskite ceramic based on complex materials.   17. Equipment according to at least one of claims 8, 9 or 11, characterized in that said at least one membrane selectively permeable to hydrogen and / or oxygen is of the mixed conduction type, ionic and electronic and / or electrolytic type. 20   Equipment according to claim 11, characterized in that the selectively hydrogen-permeable membranes (72) installed in the extraction chamber (18) form a pallet stack (70), and that each The puck comprises a thin metal layer deposited on a disk-shaped porous substrate encased in a sealing ring, a gas extraction pipe (74) pierced in the ring and joining the substrate.   19. Equipment according to at least one of claims 8, 9 or 11, characterized in that the membranes selectively permeable to oxygen and / or hydrogen installed in the treatment chamber are in the form of gloves fingers (80). 81) and / or radial lamellae and / or honeycombs. 15-40-   20. Equipment according to at least one of claims 8, 9 or 11, characterized in that, when said at least one membrane selectively permeable to oxygen and / or hydrogen (80-81), installed in the treatment chamber, are of the electrolytic type, the anode (86) of each membrane (80) is connected to the cathode (89) of the contiguous membrane (81).   21. Equipment according to claim 8 or 11, characterized in that the convergent optical device (10) is an orientable parabolic mirror or a set of reflecting facets constituting a directional Fresnel mirror or a heliostats system or a set of orientable mirrors. cooperating with a fixed parabolic mirror.