Method for generating hydrogen by decomposing water through thermochemical reaction and implementation device thereof

Abstract

The invention relates to a method and a device for producing hydrogen by decomposing water molecules by thermochemical reaction using small amounts of active substances. The thermochemical reaction is initiated by solar energy having a medium concentration of up to 50 times sunlight, which may be effected by linear or parabolic concentrators.

Description

Technical Field

[0001] The present invention relates to a method and a device for producing hydrogen by decomposing water molecules by thermochemical reaction using a small amount of active substances. The thermochemical reaction is initiated by solar energy with medium concentration, which can be achieved by various methods, among which linear concentrators are of particular significance due to their low cost and rapid development. Background Art

[0002] Green hydrogen is a priority goal for today's society to achieve sustainable development goals. It is expected that in the near future, hydrogen will not only replace natural gas, but also become the most important energy carrier, widely used in a wide range of fields such as industry, transportation and household activities (Turner, J. et al., Renewable hydrogen production. International Journal of Energy Research 32, 379–407, 2008). There are many ways to produce green hydrogen using renewable energy. Due to the maturity of the relevant technology, the use of solar-powered electrolyzers is the most common method. However, due to the losses generated during the double conversion process, the yield of hydrogen converted from the initial energy (sun) through photovoltaic power generation is very low. The process of water decomposition based on solar thermal energy through thermochemical reactions is considered to be more efficient than the combination of photovoltaic processes with electrolysis or other processes such as photolysis (Perkins, C. & Weimer, AW Solar-thermal production of renewable hydrogen. AIChE Journal 55, 286–293, 2009). The thermochemical process for water splitting consists of two steps: (i) first, the reactive material is heated to a moderate temperature between 900°C and 1300°C to initiate a reduction step, releasing oxygen in an endothermic reaction; (ii) the reactive material is then contacted with water to undergo an oxidation half-reaction, releasing hydrogen in an exothermic reaction. This two-step process has been extensively studied, and a large number of reactive materials are now known that can reach the reduction temperature using concentrated solar energy (Abanades, S., Charvin, P., Flamant, G. & Neveu, P. Screening of water-splitting thermochemical cycles potentially attractive for hydrogen production by concentrated solar energy. Energy 31, 2805–2822, 2006).It is worth mentioning that the most relevant studies have demonstrated the efficiency of using concentrated solar energy for thermochemical processes (Chueh, WC et al. High-Flux Solar-Driven Thermochemical Dissociation of CO2 and H2O Using Nonstoichiometric Ceria. Science 330, 1797–1801 (2010); Muhich, CL et al. Efficient Generation of H2 by Splitting Water with an Isothermal Redox Cycle. Science 341, 540–542, 2013). The method has been experimented with different configurations, all based on highly concentrated thermal solar systems (>200 times the sun), such as central tower systems with a power of more than 2 MW and heliostats (Muhich, CL et al., A review and perspective of efficient hydrogen generation via solarthermal water splitting. WIREs Energy and Environment 5, 261–287, 2016). The reactive material is in the form of a porous monolith or granules, which, after receiving concentrated solar radiation, reaches a reduction temperature and releases oxygen within a few minutes. Subsequently, the active material is transferred or hidden away from the solar source, where it is cooled and oxidized with water. There are several invention patents aimed at producing hydrogen from solar resources using thermochemical cycles. Among them, it is worth noting the inventions WO2008113944A2 and CN112323092A, in which the reactive material becomes gaseous during the reduction process and then solidifies during oxidation. Other inventions use particles of reactive materials that circulate through different fluidized beds, undergoing redox cycles as they pass through the reactor (US20120237440A1). Or the particles are stored in tanks, heated and reduced using solar energy, and then water is passed through the same tanks, where it is reduced and hydrogen is obtained (EP11003735A, US9399575B2). Porous monolithic systems are based on solar reactors such as the invention of Goel (WO2013021397), which are used in central tower thermal solar systems. Summary of the invention

[0003] The proposed method of producing renewable hydrogen from concentrated solar energy using thermochemical cycles has the following problems:

[0004] 1. The use of large amounts of active materials (placed in different ways) requires very high concentration factors to reach the temperatures required to initiate the reduction half-reaction, which requires the use of complex and expensive systems such as heliostat fields.

[0005] 2. The use of bulk active materials results in diffusion-limited gas transport phenomena, as well as molecular recapture and hydrogen-oxygen recombination, leading to significant performance degradation.

[0006] 3. Heat is inevitably lost from the active materials to the walls of the containing reactor, resulting in a significant drop in performance.

[0007] 4. The initiation of the oxidation half-reaction generally takes several minutes, where the active material is not exposed to solar radiation and water is present, resulting in cooling of the active material. The subsequent cycle of the reduction half-reaction requires recovery of optimal energy for this process. In the process of reheating the active material, a significant loss in performance occurs because the energy that should be used to initiate the reduction of the active material is used for heating.

[0008] 5. The use of large amounts of active material results in a lack of uniformity across the material volume, resulting in temperature and concentration gradients that negatively impact system performance.

[0009] The present invention solves all the above problems and maximizes the yield of converting solar energy into chemical energy (in the form of hydrogen), approaching the theoretical limit. The solution to the problem is based on the following points:

[0010] 1. The active material is arranged in the form of a thin layer and is continuously maintained at about the optimum operating temperature. Thus, the optimum point of reduction (starting from stoichiometric material) can be reached within a few seconds.

[0011] 2. An alternative to thin layers is to use a powder containing micron-sized particles in a suitable container.

[0012] 3. Using a conventional gas pump system, the residence time is about one second, which is less than the time required for the active material to reach the optimal reduction degree. In this way, the oxygen released in the vacuum during the reduction half reaction can be discharged before the steam is input to initiate the oxidation half reaction. Similarly, the hydrogen released in this half reaction can be discharged before the next reduction-oxidation cycle begins. This reciprocating pumping of the two gases naturally minimizes recombination losses.

[0013] 4. This configuration eliminates the need for an inert atmosphere during the reduction half-reaction.

[0014] 5. Providing active materials in the form of thin layers or micron-sized powders facilitates gas transport and minimizes the recapture of molecules, thereby preventing loss of system performance.

[0015] 6. The proposed system is based on a heat sink (trampa térmica) that absorbs the solar spectrum in the active layer (either in the active material itself, or in inclusions in the form of nanoparticles, or in an adjacent layer) and limits the blackbody radiation emitted by the thermally active material by means of an infrared reflector. The confined infrared radiation is absorbed in a layer designed for this purpose close to the active material and the absorbed energy is transferred to the active layer by conduction. Through the heat sink, heat losses are minimized, thus avoiding performance losses.

[0016] 7. Arranging the active material into thin layers or micron-sized powders and using efficient heat sinks can minimize the volume of material to be heated to the optimal temperature for the reduction half-reaction. In this way, moderate concentration factors of less than 50 times the sun are sufficient. These factors can be achieved with low-cost concentrators, increasing the economic attractiveness of the method.

[0017] 8. The use of linear concentrators has several advantages (although it is not the only solution), since its geometry is suitable for assembling tubes containing active materials. The proposed hydrogen generation system can be easily expanded by connecting several individual systems to cover the entire available land area. This is particularly easy to do with linear concentrators. Due to the easy expansion, the system can be adapted to any terrain, reducing installation costs and providing hydrogen locally, avoiding transportation from centralized production plants to end users.

[0018] 9. A small portion of the concentrated solar energy is used to generate steam, which is input in pulses during the oxidation half cycle. The higher reaction rate of the steam oxidation half reaction, compared to the reaction rate obtained using liquid water and the reduction in yield due to cooling of the active materials, highly compensates for the small energy loss required to generate the steam.

[0019] 10. The oxidation half-reaction occurs very quickly because it starts at the optimum reduction temperature and at the end of the oxidation process the temperature drops slightly due to contact with the cooler vapor. The presence of a heat sink, concentrated solar radiation during the oxidation process, and the fact that the oxidation reaction is exothermic, means that the temperature of the active layer does not drop much during the oxidation half-reaction, thus avoiding performance degradation.

[0020] 11. The hydrogen released during the oxidation half-reaction is carried out of the tube by the remaining vapor. The vapor is then condensed so that the hydrogen can be easily extracted.

[0021] Therefore, in a first aspect, the present invention relates to a method for producing hydrogen by decomposing water via a thermochemical reduction-oxidation (redox) reaction, characterized in that:

[0022] a. Using active materials that can sustain redox cycles without degradation to produce hydrogen in each oxidation half-cycle, wherein the material must be partially reduced in the reduction half-cycle and fully reoxidized in the oxidation half-cycle caused by interaction with steam;

[0023] b. The production of hydrogen is carried out continuously in a two-step cycle. The first step is to heat the active material in a vacuum to partially reduce it, thereby releasing oxygen. The second step is to input steam pulses to oxidize the active material and release hydrogen.

[0024] c. The method is carried out in a solar concentrating system, the system comprising a collector for focusing solar energy, a receiver for utilizing the concentrated solar energy system, and an absorbent for converting solar energy into thermal energy;

[0025] d. During the reduction and oxidation steps, the active material is maintained at an elevated temperature, wherein the material may act as a heat sink or be bonded to a heat sink to maximize the energy it receives from the heat sink by thermal conduction;

[0026] e. confining the energy emitted in the form of infrared radiation within the receiver within the system by a heat sink to minimize performance losses, wherein preferably the heat sink is based on a layer on the receiver that is capable of reflecting infrared radiation emitted by the thermally active material, and wherein the infrared radiation confined by the heat sink is absorbed in a region close to the active material to promote heating of the active material by thermal conduction;

[0027] f. The process control system controls the steps of each cycle by regulating the timing of each step by opening and closing the steam inlet and separating the exhaust gas in each step;

[0028] g. Before the steam of the next redox cycle enters, the oxygen produced in the reduction reaction and the hydrogen produced in the oxidation reaction are discharged from the reactor separately without mixing;

[0029] h. The solar concentration system includes an evaporator, which is preferably part of the receiver and is powered by a small portion of the solar energy collected by the collector.

[0030] Preferably, the solar concentrator is a linear concentrator with a moderate concentration of up to 50 times the sunlight, such as a parabolic trough concentrator or a Fresnel lens based concentrator, but other linear concentrators are not excluded.

[0031] Preferably, the solar concentrator is a parabolic dish concentrator with a medium to high concentration greater than 50 times the sunlight.

[0032] Regarding active materials, there are many suitable materials that can support these redox cycles. With proper treatment, these materials can be made to not degrade as the redox cycles proceed. Many metal oxides stand out, such as ceria, ferrites, zinc oxide, tin oxide, etc., or new materials based on perovskites.

[0033] Preferably, the active material is provided as a thin layer which is adhered to a substrate inside the solar receiver. If the active material does not act as a heat absorber (of the solar spectrum), the substrate is modified in the area where the active material is adhered so that it acts as a heat absorber. Furthermore, the substrate has a layer which is able to absorb the infrared radiation emitted by the thermally active material.

[0034] Preferably, the active material is provided in the form of a micron-sized powder in a thin tube within the solar receiver. If the active material in powder form does not act as a heat absorber (of the solar spectrum), the material will be doped, inclusions of a heat absorber material will be added, or the tube will be coated with a layer capable of absorbing the solar spectrum. In addition, the tube should be coated with a layer capable of absorbing the infrared radiation emitted by the thermally active material, or inclusions should be added that absorb such radiation.

[0035] Preferably, the active material is arranged in a porous container capable of retaining the active material in micron-sized powder form. Thus, the gas is transported through the porous material rather than being forced through the tube. If the active material in powder form does not act as a heat absorber (of the solar spectrum), the material will be doped, inclusions of heat absorber material will be added, or the container will be coated with a layer capable of absorbing the solar spectrum. In addition, the container wall will be coated with a layer capable of absorbing the infrared radiation emitted by the thermally active material, or inclusions will be added that absorb such radiation.

[0036] Another aspect of the present invention relates to a device for obtaining hydrogen by the above method, comprising:

[0037] a. A solar concentrator comprising a collector (110), a transparent receiver (101) and a heat absorber;

[0038] b. an evaporator (102) having a vapor inlet (103) toward a receiver, wherein the evaporator is supplied by a small portion of the solar energy collected by the collector;

[0039] c. an active material (104) disposed within the receiver (101) and acting as a heat sink or in thermal contact with a heat sink;

[0040] d. a heat sink that confines the heat within a receiver, wherein the receiver is coated with a layer (109) that reflects infrared radiation emitted by the thermally active material;

[0041] e. A gas outlet (106) controlled by a process control system;

[0042] f. a two-way gas discharge system, formed by a gas pump (111) for discharging hydrogen and water flows generated in the receiver and another gas pump (112) for discharging oxygen flows generated in the receiver;

[0043] g. An automatic process control system configured to synchronize the opening and closing of valves to properly input steam pulses (102) and properly exhaust hydrogen and oxygen through pumps (111) and (112).

[0044] Preferably, the solar concentrator is a linear concentrator (100) with a medium concentration of up to 50 times the sunlight, such as a parabolic trough concentrator or a Fresnel lens-based concentrator (110), but other linear concentrators are not excluded, and the active material is arranged in a thin layer form, or in a micron-sized powder form in a capillary, or in a porous container, as described above.

[0045] Preferably, the solar concentrator is a parabolic solar concentrator (200) having a medium to high concentration greater than 50 times the sunlight, such as a parabolic dish collector (210), but other parabolic concentrators are not excluded, and the active material is arranged in a thin layer form, or in a micron-sized powder form in a capillary tube, or in a porous container, as described above.

[0046] Preferred embodiment 1

[0047] A preferred embodiment of the invention of a device for producing hydrogen by means of a thermochemical water splitting cycle will be described below with reference to the accompanying drawings.

[0048] 1. The core of this embodiment of the invention is the receiver (101) and its components for generating and importing steam, collecting radiation, trapping heat, initiating thermochemical reactions and exhausting gases. Figure 1 Schematically shown is a receiver (101) operating with a linear collector (110). The collector may be a parabolic trough or a Fresnel collector, but any other linear collector is not excluded. Figure 2 The receiver (101), the heat sink (a thin layer of active material (104) axially disposed on a substrate (105)), the gas exhaust system (106), the steam generator (102) and the steam pulse input system (103) are shown in detail.

[0049] 2. The tubular receiver (101) is defined by a tube (109) of solar spectrum transparent material coated with a material capable of reflecting infrared radiation generated by the heat absorbing material, thereby forming a heat sink.

[0050] 3. The absorber is located on the axis of the tube, corresponding to the position of maximum solar concentration. The absorber comprises a substrate (105) on which is deposited an active material (104) capable of supporting thermochemical reactions. If the active material (104) is not a perfect absorber of the solar spectrum, inclusions in the form of nanoparticles (e.g., plasma particles) may be added, or a layer of adjacent absorber material may be added for this purpose. The substrate (105), in addition to being a mechanical support and heat storage layer, comprises an infrared radiation absorbing layer. In this way, the energy stored in the heat sink is ultimately absorbed in the vicinity of the active layer and transferred to the active layer by conduction.

[0051] 4. The device comprises a steam generator (102) and a system for inputting steam pulses in a controlled manner to activate the oxidation half-reaction. The steam generator uses a small portion of the collected light power provided by the linear concentrator. The steam generator is connected to the pipe through a fast opening valve to allow the provision of pulses (103) of controllable duration and pressure.

[0052] 5. The gas is discharged from the gas outlet (106) at the end opposite to the steam generator (103). Figure 4 ). Figure 5 A schematic diagram of separation by pulsed gas is shown. The oxygen produced is discharged through a valve into a pump system, which allows the oxygen produced in the reduction half reaction to be discharged by keeping the tube in a vacuum state (112). The hydrogen produced by the oxidation half reaction is discharged together with the excess steam by connecting the tube to the pump system, and the pump system allows the discharge of a mixture of steam and hydrogen (111). The expansion and cooling of the mixture causes the steam to condense. Therefore, the hydrogen is finally mixed with water and carried to a suitable reservoir by a conventional fluid pump to facilitate easy separation of liquid water and hydrogen.

[0053] 6. The automatic process control system can synchronously open and close the valve, correctly input steam pulses, and correctly discharge hydrogen and oxygen ( Figure 5 ), minimizing their mixing and thus reducing associated losses.

[0054] Implementation method 2.

[0055] A preferred embodiment of the invention of a device for producing hydrogen by means of a thermochemical water splitting cycle will be described below with reference to the accompanying drawings.

[0056] 1. The core of this embodiment of the invention is the receiver (101) and its components for generating and inputting steam, collecting radiation, heat capture, initiating thermochemical reactions and exhausting gases. Figure 1 Schematically shown is a receiver (101) operating with a linear collector (110). The collector may be a parabolic trough or a Fresnel collector, but any other linear collector is not excluded. Figure 3The tubular receiver (101), the heat absorber in the form of a micron-sized powder within a transparent tube (107), the gas exhaust system (106), the steam generator (102) and the steam pulse input system (103) are shown in detail.

[0057] 2. The tubular receiver (101) is defined by a tube (109) of solar spectrum transparent material. The tube (109) is coated with a material capable of reflecting infrared radiation generated by the heat absorbing material, thereby forming a heat sink.

[0058] 3. The absorber is located on the axis of the tube, corresponding to the position of maximum solar concentration. The absorber comprises a transparent thin tube filled with an active material (107) capable of supporting thermochemical reactions. If the active material is not a perfect absorber of the solar spectrum, inclusions in the form of nanoparticles (e.g., plasma particles) can be added, or a layer of adjacent absorber material can be added for this purpose. The transparent tube, in addition to serving as a mechanical support and heat accumulator, also includes a layer of infrared radiation absorber. In this way, the energy stored in the heat sink is eventually absorbed in the vicinity of the active powder and transferred to the active powder by conduction.

[0059] 4. As in the preferred embodiment 1, the device comprises a steam generator (102), a pulse input system (103) and a gas outlet (106) ( Figure 3 ). In addition, it also includes Figure 5 An automatic control system for separating gases by pulses is shown.

[0060] Implementation 3

[0061] A preferred embodiment of the invention of a device for producing hydrogen by means of a thermochemical water splitting cycle will be described below with reference to the accompanying drawings.

[0062] 1. The core of this embodiment of the invention is the receiver (101) and its components for generating and inputting steam, collecting radiation, heat capture, initiating thermochemical reactions and exhausting gases. Figure 1 Schematically a receiver (101) is shown operating with a linear collector (110). The collector may be a parabolic trough or a Fresnel collector, but any other linear collector is not excluded. Figure 4 Shown in detail are the receiver (101), the heat sink comprising active material (108) within a porous container axially disposed on a substrate (105), the gas exhaust system (106), the steam generator (102) and the steam pulse input system (103).

[0063] 2. The tubular receiver (101) is defined by a tube (109) of solar spectrum transparent material coated with a material capable of reflecting infrared radiation generated by the heat absorbing material, thereby forming a heat sink.

[0064] 3. The absorber is located on the axis of the tube, corresponding to the position of maximum solar concentration. The absorber comprises a substrate (105) on which an active material (108) capable of supporting thermochemical reactions is arranged in a porous container. If the active material (108) is not a perfect absorber for the solar spectrum, inclusions in the form of nanoparticles (e.g., plasma particles) can be added, or a layer of adjacent absorber material can be added for this purpose. The substrate (105), in addition to being a mechanical support and heat storage layer, also comprises an infrared radiation absorbing layer. In this way, the energy stored in the heat sink is eventually absorbed in the vicinity of the active layer and transferred to the active layer by conduction.

[0065] 4. As in the preferred embodiment 1, the device comprises a steam generator (102), a pulse input system (103) and a gas outlet (106) ( Figure 3 ). In addition, it also includes Figure 5 An automatic control system for separating gases by pulses is shown.

[0066] Implementation 4

[0067] A preferred embodiment of the invention of a device for producing hydrogen by means of a thermochemical water splitting cycle will be described below with reference to the accompanying drawings.

[0068] 1. The core of this embodiment of the invention is the receiver (201) and its components for generating and inputting steam, collecting radiation, heat capture, initiating thermochemical reactions and exhausting gases. Figure 6 Schematically shown is a receiver (201) operating with a parabolic collector (210). The collector may be a parabolic dish collector, but any other parabolic collector is not excluded. Figure 6 The receiver (201), the heat sink (a thin layer of active material (104) axially disposed on a substrate (105)), the gas exhaust system (106), the steam generator (102) and the steam pulse input system (103) are shown in detail.

[0069] 2. The tubular receiver (201) is defined by a tube (109) of solar spectrum transparent material. The tube (109) is coated with a material capable of reflecting infrared radiation generated by the heat absorbing material, thereby forming a heat sink.

[0070] 3. The absorber is located in the area corresponding to the position of maximum solar concentration. The absorber comprises a substrate (105) on which is deposited an active material (104) capable of supporting thermochemical reactions. If the active material (104) is not a perfect absorber for the solar spectrum, inclusions in the form of nanoparticles (e.g., plasma particles) can be added, or a layer of adjacent absorber material can be added for this purpose. The substrate (105), in addition to being a mechanical support and heat storage layer, also comprises an infrared radiation absorbing layer. In this way, the energy stored in the heat sink is eventually absorbed in the vicinity of the active layer and transferred to the active layer by conduction.

[0071] 4. As in the preferred embodiment 1, the device comprises a steam generator (102), a pulse input system (103) and a gas outlet (106) ( Figure 3 ). In addition, it also includes Figure 5 An automatic control system for separating gases by pulses is shown.

[0072] 5. The heat absorber of this embodiment can be changed according to the configuration of the heat absorber of Embodiment 2 or Embodiment 3. As in Embodiment 2, the active material is provided as a micron-sized powder in a very thin tube (107), or as in Embodiment 3, the active material is provided in a porous container on a substrate (108).

[0073] Industrial Applicability

[0074] 1. It can replace or complement (as the case may be) the combination of photovoltaic energy plus electrolyzers to produce hydrogen. In addition, it can replace all other ways to produce green hydrogen (photolysis, biological systems, etc.).

[0075] 2. It can complement photovoltaic energy because hydrogen is a vector energy that can store energy for future use, which photovoltaic energy cannot do.

[0076] 3. It allows the use of fuel cells to replace vehicle batteries, especially for heavy vehicles and vehicles with long distance (highway) driving.

[0077] 4. In addition to being able to harness solar energy through hydrogen production (particularly useful for road transport), the biggest advantage of this invention is that hydrogen can be produced locally through a distributed generation model, rather than the current centralized energy production model. This is no small problem considering that the biggest problem in the transition to a hydrogen-based economy is the difficulty in transporting hydrogen cheaply and efficiently, as hydrogen cannot be liquefied at room temperature. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1. Schematic diagram of a linear concentrating system (100). In the case of the preferred embodiment 1, there is a parabolic trough collector (110) and a tubular receiver (101) with a heat absorber plate.

[0079] Figure 2 . Receiver (101) of a linear concentrator system according to preferred embodiment 1. It comprises a tube (109) transparent to the solar spectrum, on which there is a layer for reflecting infrared radiation emitted by the heat absorber. The heat absorber material comprises a thin layer (104) of active material on a substrate (105). The area of ​​the substrate (105) in contact with the active material can effectively absorb infrared radiation. If the active material does not effectively absorb the solar spectrum, an adjacent layer is provided for this purpose. The steam generator (102) and its steam inlet (103) are shown in red, and the gas exhaust area and its outlet (106) are shown in green.

[0080] Figure 3 . Receiver (101) of a linear concentrator system according to preferred embodiment 2. It comprises a tube (109) transparent to the solar spectrum, on which there is a layer for reflecting infrared radiation emitted by the heat absorber. The active material is arranged in a thin transparent tube (107). The transparent tube is effective in absorbing infrared radiation. If the active material is not effective in absorbing the solar spectrum, a layer is arranged on the thin tube (107) for this purpose. The steam generator (102) and its steam inlet (103) are shown in red, and the gas exhaust zone and its outlet (106) are shown in green.

[0081] Figure 4 .A receiver (101) for a linear concentrator system according to preferred embodiment 3. It comprises a tube (109) transparent to the solar spectrum, on which there is a layer for reflecting infrared radiation emitted by a heat absorber. The active material is arranged in a porous container (108) on a substrate 105. The porous container (108) allows solar radiation and infrared radiation to pass through in addition to allowing the free flow of gas. The area of ​​the substrate (105) in contact with the active material can effectively absorb infrared radiation. If the active material cannot effectively absorb the solar spectrum, an adjacent layer is provided for this purpose. The steam generator (102) and its steam inlet (103) are shown at one end, and the gas exhaust area and its outlet (106) are located at the other end.

[0082] Figure 5 . Schematic diagram of the components of the pulse process and its arrangement. The solar collector (110, 210) concentrates the energy on the receiver (101, 102). The valve is connected to the actuator of the control system and opens or closes at each pulse, thereby separating the hydrogen from the water outlet (111) and the oxygen outlet (112).

[0083] Figure 6. Schematic diagram of a parabolic concentrating system (200). In the case of the preferred embodiment 4, there is a parabolic dish collector (210) and a tubular receiver (201) with a heat absorber plate.

[0084] Figure 7 . Receiver (201) of a parabolic concentrator system according to preferred embodiment 4. It comprises a tube (109) transparent to the solar spectrum, on which there is a layer for reflecting infrared radiation emitted by the heat absorber. The heat absorber material comprises a thin layer (104) of active material on a substrate (105). The area of ​​the substrate (105) in contact with the active material can effectively absorb infrared radiation. If the active material does not effectively absorb the solar spectrum, an adjacent layer is provided for this purpose. The steam generator (102) and its steam inlet (103) are shown in red, and the gas exhaust area and its outlet (106) are shown in green. Example

[0085] We developed laboratory equipment to characterize the behavior of redox-active materials in reduction and oxidation half-cycles. We developed a simulator that takes into account fluid transport, heat absorption and transfer, and the chemical reactivity of the active materials, using our measured data on different materials and published experimental data on ceria (Bulfin, B. et al. Analytical Model of CeO2 Oxidation and Reduction. J. Phys. Chem. C 117, 24129–24137 (2013)). The simulator shows that the principles underlying the invention can be used using ceria as a model material:

[0086] i. The heat sink allows the temperature to be reached that is suitable for the reduction of ceria. Therefore, under the maximum irradiance conditions (irradiance of 1 kW / m 2 , the concentration is 30 times that of sunlight), and the temperature can exceed 1400K.

[0087] ii. Taking into account the volume constraints of the receiver tube, which is 10 cm in diameter and 1 m long, the minimum pumping rate is 6 l / h, the maximum pressure is 0.1 bar, and the characteristic residence time of the gases (oxygen in the reduction step and steam plus hydrogen in the oxidation step) is about 1 second.

[0088] iii. The energy required for the evaporator is less than 1% of the energy collected by the collector.

[0089] iv. In the reduction and oxidation steps, the thermochemical model describing the kinetics has no diffusion limitations, the reduction and oxidation characteristic times of the active materials are 1 second, and the limitations are less than 5%.

[0090] v. During the oxidation step, the temperature drop of the active material is small because the film coefficient value is 10Wm -2 K -1 In the case of a pulse of 1 second, the radiation term and the exothermic reaction compensate for the convection loss. During the cycle, the system finds an equilibrium temperature above 1200K, which allows the material to be activated.

[0091] Ceria is used as a model material but does not exclude the use of other active materials with similar or superior properties.

Claims

1. A method for producing hydrogen by decomposing water by a thermochemical reduction-oxidation reaction (redox), characterized in that: a. Using an active material capable of sustaining a redox cycle to produce hydrogen in each oxidation half-cycle, wherein the material must be partially reduced in the reduction half-cycle and fully reoxidized in the oxidation half-cycle caused by interaction with steam; b. Hydrogen production is carried out continuously in a two-step cycle: the first step is to heat the active material in a vacuum to partially reduce it, thereby releasing oxygen, and the second step is to input steam pulses to oxidize the active material and release hydrogen; c. The method is carried out in a solar concentrating system, the system comprising a collector for focusing solar energy, a receiver for utilizing the concentrated solar energy system, and an absorber for converting solar energy into thermal energy; d. During the reduction and oxidation steps, the active material is maintained at an elevated temperature, wherein the material may act as a heat sink or be bonded to a heat sink to maximize the energy it receives from the heat sink by thermal conduction; e. The energy emitted in the receiver in the form of infrared radiation is confined within the system by the heat sink and absorbed in the area close to the active material; f. The steps of each cycle are controlled by a process system that regulates the timing of each step by opening and closing the vapor inlet and separating the exhaust gas in each step; g. Before the steam of the next redox cycle enters, the oxygen produced in the reduction reaction and the hydrogen produced in the oxidation reaction are discharged from the reactor; h. The solar concentrator system includes an evaporator.

2. The method for producing hydrogen as claimed in claim 1, wherein the solar concentrator is a linear concentrator having a medium concentration of up to 50 times the sunlight.

3. The method for producing hydrogen as claimed in claim 2, wherein the concentrator is a parabolic trough concentrator or a Fresnel lens-based concentrator.

4. The method for producing hydrogen as claimed in claim 1, wherein the solar concentrator is a parabolic dish concentrator having a medium to high concentration greater than 50 times that of sunlight.

5. A method for producing hydrogen as claimed in any one of claims 1 to 4, wherein the active material is provided in a thin layer which is adhered to a substrate inside a solar receiver.

6. The method for producing hydrogen as claimed in any one of claims 1 to 4, wherein the active material is provided in a thin tube in a solar receiver in the form of a micron-sized powder.

7. The method for producing hydrogen according to any one of claims 1 to 4, wherein the active material is disposed in a porous container capable of retaining the active material in a micron-sized powder form.

8. A method for producing hydrogen as claimed in any one of the preceding claims, wherein the heat sink is a layer on the receiver capable of reflecting infrared radiation emitted by the thermally active material, and wherein the infrared radiation confined by the heat sink is absorbed in an area close to the active material to promote heating of the active material by thermal conduction and minimize production losses.

9. A method for producing hydrogen as claimed in any one of the preceding claims, wherein the evaporator is part of a receiver and is fed by a small portion of the solar energy collected by the collector.

10. An apparatus for obtaining hydrogen by the method according to any one of the preceding claims, comprising: a. A solar concentrator comprising a collector (110), a transparent receiver (101) and a heat absorber; b. an evaporator (102) having a vapor inlet (103) leading to a receiver; c. an active material (104) disposed inside the receiver (101); d. a heat sink that confines heat to a receiver (101), wherein infrared radiation emitted within the receiver is absorbed in an area close to the active material; e. A gas outlet (106) controlled by a process control system; f. a two-way gas discharge system, formed by a gas pump (111) for discharging hydrogen and water flows generated in the receiver and another gas pump (112) for discharging oxygen flows generated in the receiver; g. An automatic process control system configured to synchronize the opening and closing of valves to properly input steam pulses (102) and properly exhaust hydrogen and oxygen through pumps (111) and (112).

11. The device for producing hydrogen as claimed in claim 10, wherein the solar concentrator is a linear concentrator (100) with a medium concentration of less than 50 times that of sunlight, and the active material is arranged in the form of a thin layer, or in the form of a micron-sized powder in a thin tube, or in a porous container.

12. The device for producing hydrogen as claimed in claim 10, wherein the solar concentrator is a parabolic solar concentrator (200) having a medium to high concentration greater than 50 times that of sunlight, and the active material is arranged in the form of a thin layer, or in the form of a micron-sized powder in a thin tube, or in a porous container.

13. The hydrogen production device according to any one of claims 10 to 12, wherein the receiver is coated with a layer (109) capable of reflecting infrared radiation emitted in the receiver.

14. The device for producing hydrogen according to any one of claims 10 to 13, wherein the evaporator (102) is a part of a receiver and is supplied by a small part of the solar energy collected by the collector.

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