Method and device for producing hydrogen by means of water dissociation by thermochemical reactions under (quasi-)isothermal conditions using different power sources

Abstract

The present invention relates to a method for producing hydrogen by means of thermochemical water dissociation cycles under (quasi-)isothermal conditions, wherein said method comprises arranging a large amount of active material (104) inside a reaction volume (109) of a reactor (103); heating the active material (104), reducing the active material (104) and generating oxygen in the reaction volume; evacuating the oxygen produced via a first evacuation path (111) of the outlet (106) of the reactor (103); injecting water into the reaction volume (109) of the reactor, oxidating the active material (104) and producing hydrogen; evacuating the hydrogen produced via a second evacuation path (112) of the outlet (106) of the device (100); and separating the evacuated hydrogen and remaining water. The invention further relates to a device for producing hydrogen.

Description

[0001] Method and device for producing hydrogen by dissociation of water through thermochemical reactions under (quasi-) isothermal conditions using different energy sources

[0002] TECHNICAL SECTOR

[0003] The present invention relates to a method and device for the production of hydrogen through the dissociation of water molecules using thermochemical reactions under (quasi-)isothermal conditions. Thermochemical reactions are induced by energy obtained from various sources other than solar energy. Electrical energy is particularly relevant due to its widespread availability, and waste heat from various industrial processes is also important due to the possibility of recovering it in the form of hydrogen.

[0004] BACKGROUND OF THE INVENTION

[0005] At the time of writing this patent, green hydrogen is positioned as the most relevant energy vector for the near future. Green hydrogen will not only replace natural gas, but will also be used in a wide range of applications within transportation and industry, as well as energy distribution and storage applications (Turner, J. et al. Renewable hydrogen production. International Journal of Energy Research 32, 379–407,

[0006] 2008).

[0007] Green hydrogen is virtually the only realistic option for drastically reducing polluting gas emissions and thereby achieving sustainability goals. The most widely used green hydrogen production method today, due to its advanced stage, is based on the use of electrolyzers powered by electricity from renewable sources, such as photovoltaics or wind power.

[0008] However, when the main source is solar radiation, the efficiency of the photovoltaic and electrolyzer array is very low. On the other hand, methods based on thermochemical redox reactions require a heat source. These methods exploit solar energy capture much more efficiently (Perkins, C. & Weimer, A.W. Solar-thermal production of renewable hydrogen. AIChE Journal 55, 286–293,

[0009] (2009). Methods based on thermochemical reactions dissociate the water molecule into oxygen and hydrogen through a two- or multi-step process.

[0010] First, the active material, capable of supporting redox reactions, is reduced at high temperatures, generally between 1,100 and 2,300 K, releasing oxygen, according to the reduction half-reaction, and leaving oxygen vacancies in its structure. In a second step, the material is oxidized in the presence of water, according to the oxidation half-reaction, releasing only hydrogen.

[0011] This thermochemical process has been extensively studied with a large number of active materials, where the reduction temperature is reached by heating, ideally 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). The review article by Oudejans et al. (Oudejans, D., Offidani, M., Constantinou, A., Albonetti, S., Dimitratos, N., Bansode, A., 2022. A Comprehensive Review on Two-Step Thermochemical Water Splitting for Hydrogen Production in a Redox Cycle. Energies 15, 3044) shows the development of hydrogen production from thermochemical reactions, highlighting the methodologies and experimental facilities used.

[0012] The methods that show the best efficiencies for hydrogen production from solar energy (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 H2by Splitting Water with an Isothermal Redox Cycle, Science 341 , 540-542, 2013) are designed to work mainly with high solar concentration systems, such as central tower or parabolic dish systems (Muhich, CL et al. A review and perspective of efficient hydrogen generation via solar thermal water splitting, WIREs Energy and Environment 5, 261-287, 2016).

[0013] These methodologies have been tested in the laboratory and, in some cases, in solar concentration towers, with different heating systems of an active material that can be a porous monolith, particles, or even have a gaseous phase (Mao, Y., Gao, Y., Dong, W., Wu, H., Song, Z., Zhao, X., Sun, J., Wang, W., 2020. Hydrogen production via a two-step water splitting thermochemical cycle based on metal oxide - A review, Applied Energy 267, 114860). In certain oxides, it has been shown that nanosized material improves the rate of the reduction reaction (Bulfin, B., Lowe, A.J., Keogh, K.A., Murphy, B.E., Lübben, O., Krasnikov, S.A., Shvets, I.V., 2013, Analytical Model of CeOs Oxidation and Reduction. J. Phys. Chem. C 1 17, 24129-24137). Among the patented inventions involving thermochemical cycles, the following stand out: W02008113944A2 and CN112323092A, where the reactive material is brought to the gas phase during the reduction.

[0014] Other devices circulate reactive material particles in a fluidized bed (US20120237440A1 ), or mechanically move the particles from a reduction zone with direct irradiation to a storage or oxidation region (EP11003735A, US9399575B2). Although most inventions seek to heat the active material with solar energy, there are systems that heat the active material with microwaves (WO2021159874A1 ) or other sources of thermal energy, such as laboratory devices (Y. Mao et al., Hydrogen production via a two-step water splitting thermochemical cycle based on metal oxide - A review, Applied Energy, vol. 267, p. 114860, Jun. 2020). These thermal energy sources can be powered by electricity. The referenced devices that operate with sources other than solar are far from the design of the present invention, and have very low efficiencies.

[0015] It's important to mention that most of the above systems operate at different temperatures for the reduction and oxidation half-reactions; that is, they require a thermal jump that occurs when the material is cooled during oxidation in the presence of water. The energy consumed in reheating the material during reduction significantly reduces efficiency.

[0016] On the other hand, publications that study cycles under isothermal conditions (same temperature in both half-reactions) or quasi-isothermal conditions (similar temperatures in both half-reactions) show a theoretical improvement in efficiency (Al-Shankiti, I., Ehrhart, BD, Weimer, AW, 2017. Isothermal redox for H2O and CO2 splitting - A review and perspective, Solar Energy, Advances in Solar Thermochemistry 156, 21-29).

[0017] The problem detected in (quasi-)isothermal cycles is the large amount of water needed to oxidize the material, which implies a high energy expenditure to vaporize the water, making it necessary to use heat recovery systems and / or alternatives capable of minimizing water consumption.

[0018] A technology that solves this problem has been described in application ES2932843. This application describes a method and device for producing hydrogen from thermochemical reactions, using concentrated solar power as the primary source. However, the solution described in this application has several limitations imposed by the use of solar energy, such as the need for a small amount of active material.

[0019] Another significant limitation of the solution described in patent ES2932843 is the thermal trap. The use of solar energy requires the thermal trap to allow light to enter, considerably limiting its purpose of retaining the blackbody radiation emitted by the hot active material. This translates into energy losses that decrease the device's efficiency. This limitation becomes more evident as working at higher temperatures, because the emission spectrum of the hot material increasingly resembles that of the sun (the surface of the sun has a temperature of 5,772 K). That is, as working at higher temperatures, the thermal trap releases more energy.In practice, this means that the reactor's operating temperature is considerably limited by this problem. At lower temperatures, the active material is reduced less, and therefore, less hydrogen is produced per cycle (in other words, the reactor's power is lower). This problem is exacerbated by the limited use of active material. The synergistic effect of both problems means that reactor scalability is compromised.

[0020] In short, while a solar device is the most attractive technology in the medium and long term, other options are needed that are more feasible in the short term and may be equally attractive in certain market niches and / or during the transition to a hydrogen-based economy. Another scenario in which these systems can be very interesting is when there is an excess of some type of energy (for example, a surplus of electricity production) that needs to be stored.

[0021] SUMMARY OF THE INVENTION

[0022] The present invention relates to a method and device that exploits the characteristics of various energy sources, other than solar energy, to optimize reduction and oxidation processes, improving the intrinsic efficiency of current methods based on thermochemical reactions.

[0023] In the present invention, novel embodiments of devices and methods are described that improve upon, either complement or replace, other configurations known in the prior art, utilizing various energy sources other than solar energy to produce hydrogen through thermochemical water dissociation cycles, with improved efficiency. Thus, according to the present application, each of the energy sources other than solar energy can be defined as a non-solar energy source.

[0024] Thus, in a first aspect of the invention, a process is described for producing hydrogen by means of thermochemical water dissociation cycles. Said thermochemical cycles are carried out under quasi-isothermal conditions (i.e., at an approximately constant temperature), using an active material.

[0025] As used herein, an active material is defined as one capable of cyclic reduction and oxidation, producing hydrogen in each oxidation half-cycle. That is, said material is partially reduced in the reduction half-cycle, consuming energy in the form of heat, and oxidized in the oxidation half-cycle due to interaction with water, releasing or producing hydrogen.

[0026] Said method then comprises the following steps: a) placing the active material inside a reaction volume of a reactor; b) heating the reaction volume, reducing the active material and generating oxygen in the reaction volume; c) evacuating the oxygen produced through a first evacuation path at the reactor outlet; d) injecting water into the reaction volume of the reactor, oxidizing the active material and producing hydrogen; and e) evacuating the hydrogen produced through a second evacuation path at the device outlet.

[0027] When heating the reaction volume in step b, the hot active material in the reaction volume can emit heat by radiation. However, up to 99% of the heat emitted by the hot active material is returned to the reaction volume by a heat trap, preventing its loss and thus improving the efficiency of the process. In other words, the reaction volume is heated by a combined input of heat from a primary non-solar energy source and an input of energy recovered from the radiation losses of the active material.

[0028] According to this procedure, it is especially important to have a (nearly) constant temperature in the reactor, so that the oxidation half-reaction occurs very quickly, since it starts at the optimum reduction temperature and at the end of the oxidation process the temperature has dropped little due to contact with the injected water.

[0029] In a second aspect of the invention, a device is described for producing hydrogen by means of thermochemical cycles of water dissociation under quasi-isothermal conditions from an active material capable of sustaining redox cycles with hydrogen production.

[0030] Said device comprises: a. a reactor comprising, in turn, a reaction volume configured to contain an active material and carry out the thermochemical water dissociation cycles; b. an inlet, connected to a water injection system, configured to introduce water into the reactor; c. a gas outlet, comprising a two-way system, where: i. a first evacuation path is configured to evacuate oxygen from the reaction volume of the reactor;

[0031] i. a second evacuation path is configured to evacuate the hydrogen produced from the reaction volume of the reactor; d. a thermal trap configured to confine the energy emitted in the form of radiation within the reactor and absorb it in an area close to the reaction volume of the reactor, comprising at least one radiation reflector, located inside an airtight container, configured to prevent, in use, the escape of the radiation emitted by the hot active material; e. a non-solar energy source configured to heat, in use, an active material arranged in the reaction volume of the reactor; and f. a process control system comprising a set of valves and pumps configured to regulate the stages of the redox cycle, synchronizing the opening and closing of the water injection system and the exit of the generated gases.According to the present invention, non-solar energy sources are understood to be those sources, among which the following may be mentioned, without ruling out other sources:

[0032] • Electricity (e.g. by resistive or inductive heating);

[0033] • High-temperature industrial heat waste (e.g., hot fluids from furnaces, or any industrial process);

[0034] • Microwave;

[0035] • Nuclear;

[0036] • Geothermal energy;

[0037] • Infrared radiation;

[0038] • Combinations of the above.

[0039] By using any of these non-solar energy sources, the heat trap can feature a highly efficient reflector, capable of reflecting up to 99% of the radiation emitted by the hot material, surrounding the reactor's reaction volume, considerably limiting radiation losses. The presence of the non-solar energy source during oxidation, the heat trap, and the fact that oxidation reactions are exothermic, means that the temperature of the active material does not drop significantly during the oxidation half-reaction, thereby avoiding a decrease in performance. When solar energy is used as the primary source, temperatures of up to 2,000 K are reached in the best-case scenario, limited by the available solar radiation and because not all the energy lost through radiation can be recovered. Therefore, an additional advantage of these non-solar energy sources is that they allow working at higher temperatures, up to 2.500 K, and limited only by the materials, with the consequent improvement in the intrinsic efficiency of the thermochemical cycle.

[0040] In addition to the improved ability of the heat trap according to the present invention to retain the radiation emitted by the hot material, it is also possible to reduce heat conduction and convection losses provided by the hermetic container, preferably in a vacuum. As a result, both the device and the process described in the present invention have much lower heat losses than known solutions for harnessing solar energy.

[0041] On the one hand, a system that replaces solar energy with a non-solar source retains several of the characteristics of other current solutions based on solar energy, for example:

[0042] 1. Arrange an active material, whether in the form of a layer, granules, or porous solid, and maintain it at a roughly constant temperature. This results in a quasi-isothermal process, close to the optimal operating temperature. Thus, starting from stoichiometric material, the optimal reduction point can be reached in a time of approximately 60-600 s, thus reducing efficiency losses. Additionally, these arrangements promote gas transport and reduce the re-entrapment of molecules, avoiding losses in system performance.

[0043] 2. Use conventional gas pumping systems, with residence times on the order of seconds, which are shorter than the time required to achieve the optimal degree of reduction of the active material. In this way, the oxygen released in vacuum during the reduction half-reaction can be evacuated before the water is admitted (by injecting a controlled amount of liquid or vaporous water) to induce the oxidation half-reaction. Likewise, the hydrogen released in this half-reaction can be evacuated before the next reduction-oxidation cycle begins. This alternate pumping of both gases naturally minimizes recombination losses and can avoid the use of inert atmospheres during the reduction half-reaction.

[0044] 3. Consume a small amount of water, considerably reducing losses caused by water vaporization in the oxidation half-cycle.

[0045] However, the device according to the present invention is optimized to use non-solar energy sources, which enables the invention to:

[0046] 1. By not having to allow the entry of the solar spectrum, the efficiency of the thermal trap is improved, as it reflects up to 99% of the blackbody radiation emitted by the hot active material.

[0047] 2. Use the device continuously for 24 hours a day.

[0048] 3. Work at higher temperatures (up to 2500 K in these systems against a maximum of 2000 K in the solar case), by comprising a better thermal trap and not being limited by the available solar radiation.

[0049] 4. Use a greater amount of active material in the reaction volume, which translates into greater hydrogen production capacity per cycle.

[0050] Thus, by eliminating heat losses due to radiation, intrinsic to a solution with a solar energy source, and reducing those corresponding to the water vaporization process, the process's efficiency is greater. Therefore, the solution described in the present invention can replace or function complementarily (as appropriate) to all state-of-the-art hydrogen generation methods (electrolysis, photolysis, biological systems, etc.). Additionally, the use of a higher operating temperature than that of solar solutions increases the intrinsic efficiency of thermochemical cycles.

[0051] When coupled with a fuel cell, it can replace batteries in all types of applications. Two particularly interesting cases are vehicles, especially heavy and long-distance (highway) vehicles, and self-consumption applications, where a home could even be completely disconnected from the grid (electricity and gas), as the hydrogen produced with the excess electrical energy from a photovoltaic system could cover both electricity needs in the absence of sunlight and gas for heating.

[0052] Finally, the hydrogen produced can be used, with or without purification steps, as a raw material in industry as it is today. Furthermore, it can replace natural gas in all applications where this or other hydrocarbons are used to generate heat.

[0053] The following references have been used in the figures

[0054] 100. Device

[0055] 101 . Airtight container

[0056] 102. Thermal trap

[0057] 103. Reactor

[0058] 104. Active material

[0059] 105. Reactor inlet

[0060] 106. Reactor exit

[0061] 107. Control system

[0062] 108. Non-solar energy source

[0063] 109. Reaction volume

[0064] 110. Injection system

[0065] 111. First evacuation route

[0066] 112. Second evacuation route

[0067] 201 . Resistance 301 . High temperature fluid heating medium jacket

[0068] 401 . Thermal module of electromagnetic radiation

[0069] BRIEF DESCRIPTION OF THE FIGURES

[0070] Figure 1 shows a schematic of an embodiment of the device consisting of a vacuum-sealed container, a reflector, and a reactor containing the active material in its reaction volume, through which gases circulate from an inlet and an outlet.

[0071] Figure 2 shows a diagram of a preferred embodiment of the device, where the device comprises a source of electrical energy, with heating by means of a resistor arranged around the reactor.

[0072] Figure 3 shows a schematic of an embodiment of the device, where the device comprises a thermal energy source, with heating by a high temperature fluid arranged around the reactor (in the form of a standard heat exchanger).

[0073] Figure 4 shows a schematic of an embodiment of the device, where the device comprises a non-solar electromagnetic energy source, being formed by an electromagnetic radiation thermal module that radiates the energy to the active material, heating the active material to the working temperature.

[0074] Figure 5 shows a schematic of an embodiment of the device, where the device comprises a process control system formed by an automatic control module.

[0075] DETAILED DESCRIPTION OF THE INVENTION

[0076] In a first aspect of the invention, the solution relates to a process for producing hydrogen by means of thermochemical cycles of water dissociation under (quasi-)isothermal conditions from an active material capable of sustaining redox cycles, with production of hydrogen in each oxidation half-cycle, where said material is partially reducible in the reduction half-cycle, consuming energy in the form of heat, and is oxidized in the oxidation half-cycle, due to the interaction with water.

[0077] Said method comprises the following steps: a) placing the active material (104) inside a reaction volume (109) of a reactor (103); b) heating the reaction volume (109) by means of the joint contribution of heat from a non-solar energy source (108) and heat recovered by a thermal trap (102), from the radiation losses of the active material (104), reducing the active material (104), and generating oxygen in the reaction volume; c) evacuating the oxygen produced through a first evacuation path (11 1) of the outlet (106) of the device (100); d) injecting water into the reaction volume (109) of the reactor (103), oxidizing the active material (104) and producing hydrogen; and e) evacuating the hydrogen produced through a second evacuation path (112) of the outlet (106) of the device (100).

[0078] Part of the heat emitted by the hot active material (104) during the heating of the reactor (103) in step b) can be recovered, generating a temperature increase in an absorbent arranged within the reaction volume (109).

[0079] Unlike other previous solutions, such as the one described in application ES2932843, the present invention manages to eliminate the limitations imposed by the use of solar energy.

[0080] In this way, by not requiring the entry of light into the reaction volume, up to 99% of the radiation emitted by the hot active material (104) can be recovered by the thermal trap (102) and supplied again as recovered heat in step b). This reduces energy losses, increasing the efficiency of the device. This improvement becomes more evident as working at higher temperatures. That is, as working at higher temperatures, the thermal trap (102) manages to reduce energy losses further.

[0081] Furthermore, the use of a non-solar energy source (108) as described in the present invention allows the use of a greater quantity of active material (104), due to the greater power density of these energy sources.

[0082] The total power of the reactor (103) is directly proportional to the amount of active material (104) used, so the ability to increase the amount of active material (104) used results in devices with greater power and more efficient processes. Therefore, the increase in the amount of active material (104) used, together with the improvement of the thermal trap (102) according to the present invention, substantially improves the scalability of this type of device compared to other known devices.

[0083] In a particular embodiment, the active material (104) occupies at least 75% of the reaction volume (109), allowing the transfer of gases within the reactor (103) in the remaining 25% of the reaction volume (109). Additionally, in a particular embodiment, where the active material (104) is in powder form or is a porous material, the active material (104) can be arranged occupying the entire reaction volume (109), since the space between grains or the porosity of the material itself allows adequate gas transfer.

[0084] In one embodiment, the active material (104) may act as a direct absorber of the radiation reflected by the heat trap. However, in an alternative embodiment, the active material (104) may be arranged in contact with the absorber, such that heat from the absorber is transferred to the active material (104) by thermal conduction.

[0085] Optionally, the hydrogen obtained according to the present invention and the remaining water may require additional separation and / or purification processes, for example, a water condensation process. Thus, the process may additionally comprise a step of separating the evacuated hydrogen and the remaining water.

[0086] In order to carry out the process for producing hydrogen, the solution according to the present invention relates, in a second aspect, to a device (100) for producing hydrogen.

[0087] Figure 1 shows a schematic of an embodiment of the device (100) formed by an airtight container (101), a reflector (102), and a reactor (103) containing the active material (104) within its reaction volume (109), through which the components circulate from an inlet (105) and an outlet (106).

[0088] Said reactor (103) comprises a reaction volume (109) configured to contain an active material (104) inside and carry out the thermochemical cycles of water dissociation. As indicated above, there are numerous materials that can act as active material (104), among which metal oxides can be highlighted, such as ceria, ferrites, zinc oxide, tin oxide, or more novel materials based on perovskites, without ruling out other materials. For this reason, the reactor (103) can have numerous configurations, adapting to the active material (104) used, a preferred embodiment being a tubular reactor, as shown in Figure 1 , where the active material (104) is arranged along the length of the reaction volume (109).

[0089] The reaction volume (109) is configured to arrange an active material (104) in different configurations. If the active material (104) does not act as a direct absorber of the radiation returned by the heat trap (102), the reactor (103) may further comprise a means, for example, a second material, to act as an absorber of the returned radiation, and which may also act (or not) as a support for the active material. That is, said reactor may comprise an absorbent associated with the active material (104), where said absorbent is configured to transmit the recovered heat to the active material (104) by conduction. Additionally, said absorbent is preferably an absorbent support.

[0090] Alternatively, the active material (104) can be arranged in the form of a micrometre or sub-micrometre powder. As in the method, the reaction volume (109) of the device can be configured to contain the active material (104) occupying at least 75% of the reaction volume (109). In this way, the transfer of gases within the reactor (103) is allowed in the remaining 25% of the reaction volume (109). Additionally, in a particular embodiment, where the active material (104) is in the form of powder or is a porous material, the reaction volume (109) can be configured so that the active material (104) occupies the entire reaction volume (109), since the space between grains or the porosity of the material itself allows adequate gas transfer.

[0091] If the active material (104), arranged in powder form, does not act as an absorbent, the reactor (103) comprises an absorbent, capable of absorbing the infrared radiation reflected by the thermal trap and transmitting it to the active material (104), preferably in the form of a layer, or as inclusions. An example of inclusions are nanometric particles, for example, plasmonic particles.

[0092] In another embodiment, the active material (104) may be arranged as a porous bulk material such that gases generated in the reaction volume (109) flow through the porosities. Again, if the active material (104) does not act as a direct absorbent, the reactor (103) may comprise an absorbent, such as inclusions, capable of absorbing infrared radiation reflected by the heat trap and transmitting it to the active material (104).

[0093] On the other hand, the device (100) comprises a non-solar energy source (108) which, as indicated above, is different from solar energy, and is configured to heat, in use, the active material (104) arranged in the reaction volume of the reactor (103). Preferably, the temperature reached thanks to the use of a non-solar energy source (108) is up to 2500 K, limited only by the materials. In this way, the optimum reduction temperature of the active material (104) used is set, which will remain practically constant throughout the process.

[0094] Compared to other solutions in the state of the art, the energy required to keep the reaction volume (109) hot to a certain temperature is lower, due to the joint contribution of the heat delivered by the non-solar energy source (108) and the energy recovered from the radiation emitted by the active material (104).

[0095] In a preferred embodiment, the non-solar energy source (108) is an energy source selected from the group consisting of: a resistive heat source, an inductive heat source, a source based on a hot fluid, a source of electromagnetic radiation, a nuclear energy source, a geothermal energy source and a combination of these sources.

[0096] Specifically, the device (100) for producing hydrogen may have a resistive heat source (an electrical resistor) arranged around the reactor (103). The use of electricity as a non-solar energy source (108) of the device (100) allows this device (100) to be used as a replacement for the electrolyzers that are currently used to generate green hydrogen, avoiding the limitations present in this type of equipment.

[0097] Alternatively, a heat source consisting of a high-temperature fluid contained in a jacket (301) surrounding the reactor (103) may be used, or the inverse configuration, with the hot fluid contained in a tube that is concentrically surrounded by the reactor (103). In other words, the reactor (103) would be configured in a manner equivalent to a heat exchanger to transmit the energy of a high-temperature fluid to the active material (104). In this sense, optionally, the residual heat from certain industrial processes that release some high-temperature fluid, higher than the operating temperature of the reactor (103), could be used as a non-solar energy source, which would help to optimize the existing energy resources. Therefore, it is a way of converting energy that is currently lost into hydrogen.

[0098] Another option is to obtain the hot fluid using nuclear energy as a non-solar energy source (108). The most attractive option, but not necessarily the only one, is the use of gas-cooled high-temperature reactors. These reactors can achieve very high fuel utilization rates and operate at high temperatures, which favors the production of hydrogen according to the present invention. For example, an ideal type of reactor for this purpose could be based on the concept of the Very-High-Temperature Reactor (VHTR). These reactors will have an outlet temperature of 1,000 °C, using graphite as a moderator and helium or molten salt as a coolant. An additional advantage would be the passive safety system of these devices.

[0099] Alternatively, electromagnetic radiation such as microwaves or infrared radiation could be used as a non-solar energy source (108).

[0100] The use of microwave radiation as a non-solar energy source (108) allows high temperatures to be reached, up to 2500 K, and provides additional advantages, such as the fact that the entire active material (104) is coupled into the energy field, absorbing energy directly throughout the reaction volume. This embodiment is especially interesting in the case of using ceramics as the active material (104).

[0101] The great advantage of using infrared modules as a non-solar energy source (108) is that they can be perfectly adjusted to the absorbent properties of the active material (104), considerably simplifying the process and the device (100). That is, the use of an absorbent support medium in the reaction volume in addition to the active material (104) is avoided. Another great advantage of infrared heating modules is their scalability, which is ideal for their current implementation. Finally, a combination of sources comprising a first non-solar energy source, selected from the group described above, and a second energy source can be used as a non-solar energy source (108), where, in addition to said sources, the possibility of using solar energy is included. Using more than one energy source has the advantage that systems can be designed that, depending on availability, can operate uninterruptedly.For example, one embodiment of the present invention comprises a mixed energy source using solar energy during the day and any of the other non-solar energy sources during the night.

[0102] In order to achieve a substantial improvement over other solutions in the state of the art, the device (100) comprises a thermal trap (102). Said thermal trap (102) has at least one radiation reflector, surrounding the reactor (103), so that it confines the energy emitted in the form of radiation within the reaction volume (109) of the reactor (103). In this way, radiation losses during heating of the active material (104) are avoided.

[0103] In a preferred embodiment, the heat trap (102) comprises a reflector that completely surrounds the reactor (103), preventing the energy emitted by the hot materials from being lost in the form of radiation. In another preferred embodiment, the reflector of the heat trap (102) may have a stratified structure (Bragg mirror), with a layer designed to confine and absorb infrared or visible radiation, near the active material (104), and transmitting the absorbed energy by conduction.

[0104] As mentioned above, the configuration of the thermal trap (102), according to the present invention, allows higher temperatures to be reached, up to 2,500 K, in the reaction volume (109), which translates into a faster (and greater) reduction of the active material (104), which substantially improves the kinetics of the redox reaction and decreases the mass of active material required.

[0105] Thus, in a specific case where ceña is used as the active material (104), temperatures above 1700 K can be easily reached using any of the non-solar energy sources (108) described above, which increases the reaction kinetics and the efficiency of the process. The reactor (103), as well as its casing, which forms the thermal trap (102), is in turn contained in an airtight container (101 ). This container (101 ), preferably, has a low thermal conductivity, avoiding heat loss by conduction, and provides vacuum conditions to the system, which inhibit loss by convection. According to the embodiment shown in Figure 1 , the airtight container (101 ) has a parallelepiped structure, with a cubic shape being preferable, although said configuration may vary depending on the design used for the reactor (103) and the thermal trap (102), for example, in the form of a cylinder.

[0106] Finally, the device (100) has a process control system (107). Said control system (107) is associated with a set of valves and pumps and is configured to regulate the stages of the redox cycle, synchronizing the opening and closing of the water injection system (110) and the output of the generated gases. The synchronization comprises the regulation of the time of each of the stages, opening and closing the water inlet and separating the gases generated in each stage. In this way, the oxygen generated in the reduction half-reaction and the hydrogen generated in the oxidation half-reaction are evacuated from the reactor (103) separately and without mixing, before the entry of water vapor of the next redox cycle. Specifically, the first and second evacuation paths (111, 112) may comprise one or several pumps to evacuate the compounds and, additionally, be common to both evacuation paths.

[0107] In a preferred embodiment, the control system (107) is an automatic control system, which achieves the correct admission of the steam pulses and the correct evacuation of hydrogen and oxygen.

[0108] Additionally, since the solution does not depend on solar energy, the device (100) can operate uninterruptedly 24 hours a day, which is very advantageous in all applications that require a continuous supply of hydrogen, eliminating or reducing hydrogen storage requirements.

[0109] A preferred embodiment of the present invention is described below with the aid of the attached figures: a device for hydrogen production by means of thermochemical cycles of water dissociation under (quasi-)isothermal conditions.

[0110] Figure 2 shows an embodiment of a device (100) that uses a non-solar electrical energy source (108). In this case, said non-solar energy source is formed by a heat-generating resistor (201) arranged around the reactor (103). The heat generated by the resistor is transmitted by conduction to the reaction volume of the reactor (103) and consequently to the active material (104).

[0111] Resistive heating is represented in the diagram, but other alternative means of electrical heating could also be used, such as induction heating, for which the reactor (103), or at least a part of it, must be suitable for inducing eddy currents by means of variable magnetic fields.

[0112] Alternatively, Figure 3 shows a preferred embodiment of a device (100) that uses as a non-solar energy source (108) a high-temperature fluid contained in a jacket (301) concentric to said reactor (103). Specifically, a device (100) with a tubular reactor (103) is shown, where the non-solar energy source (108) is a high-temperature fluid heating means (301) arranged around the reaction volume of said reactor (103), forming a set of concentric tubes, where the high-temperature fluid flows through the external part of the reactor (103). In another preferred embodiment, said fluid may come from an industrial process, a gas-cooled nuclear reactor, a geothermal source, without excluding other heat sources. In this way, the concentric jacket (301) transmits the heat of a high-temperature fluid by conduction to the reactor (103) and consequently to the active material (104).

[0113] A configuration that is also possible, alternatively to that shown in Figure 3, is that arrangement where the hot fluid passes through a tube, this tube being the one concentrically surrounded by the reactor (103) and in direct contact with the active material (104). Both this configuration and the previous one function as a heat exchanger, since the active material (104) receives the energy from a fluid that is at a higher temperature.

[0114] Similarly, Figure 4 shows an embodiment of a device (100) that uses as a non-solar energy source (108) an electromagnetic radiation thermal module (401). Said electromagnetic radiation thermal module (401) radiates the energy to the active material (104), heating it to the working temperature.

[0115] Examples of an electromagnetic radiation thermal module (401) may be an infrared or microwave module, without excluding other energy sources. Thus, the reactor (103), or at least a part of it, must be suitable for absorbing the radiation from the active material (104) used and converting it into heat that is transmitted to the active material (104) contained in the reactor (103).

[0116] Finally, Figure 5 shows an embodiment of the device (100), which comprises a process control system (107) formed by an automatic control module. Said automatic control module in turn comprises a set of sensors, controllers and actuators. In this way, the actuators of the control module are connected to the valves of the inlet and outlet flows of the reactor (103). Thus, the valves can be activated to open or close according to the cycles established in the process to produce hydrogen, separating the flow of oxygen generated at the exit from the flow formed by hydrogen and water.

[0117] On the other hand, Figure 5 shows how the inlet (105) is connected to a water injection system (110) and allows a flow of water to enter the reaction volume (109) of the reactor (103). In a preferred embodiment, the water injection system (110) comprises a vaporizer configured to introduce a flow of water in the form of steam. This vaporizer can be powered by energy from multiple sources, including solar energy.

[0118] According to a preferential process, that is, one with a 1:1 ratio between the number of moles of water consumed versus moles of hydrogen produced in each cycle, an intrinsic efficiency of the process can be estimated to be greater than 85%, due to the inevitable losses caused by the vaporization of water.

[0119] In a preferred embodiment, the volume available for the water injected in each cycle, that is, the volume not occupied by the active material (104) represents less than 10% of the reaction volume (109).

[0120] According to this preferred embodiment, hydrogen is produced with a low water consumption (preferably a molar ratio between water and hydrogen of 1:1, and never greater than 10:1), so that the losses to generate steam during the oxidation half-cycle are considerably reduced, which allows very high intrinsic efficiencies to be achieved. Furthermore, in another preferred embodiment, the water injection system (1 10) has a pulsed inlet. This pulsed configuration provides the system with greater control over the development of the process, allowing for better separation in the gas flows obtained during the process.

[0121] For its part, the outlet (106) is preferably a gas outlet and comprises a two-way system, a first way and a second way, where: a. the first evacuation way (11 1 ) is configured to evacuate the oxygen produced from the reaction volume (109) of the reactor (103). b. the second evacuation way (1 12) is configured to evacuate the hydrogen produced, possibly mixed with the water remaining from the oxidation, from the reaction volume (109) of the reactor (103), and

[0122] Therefore, as shown in these embodiments, there are multiple options for use as a non-solar energy source (108), other than solar energy. These configurations allow the efficiency of the system to be considerably improved with respect to the state of the art, and to compete advantageously with other solutions currently used, such as electrolyzers. Therefore, a device (100) and a method for producing hydrogen capable of operating continuously 24 hours a day are achieved, as they do not depend on solar energy, reducing heat losses due to radiation, which translates into a complement or replacement of known solutions.

Claims

CLAIMS 1. A process for producing hydrogen by means of thermochemical water dissociation cycles under (quasi-)isothermal conditions from an active material capable of sustaining redox cycles, with hydrogen production in each oxidation half-cycle, said material being partially reduced in the reduction half-cycle, consuming energy in the form of heat, and reoxidized in the oxidation half-cycle due to interaction with water, said process comprising the following steps: a) placing the active material (104) inside a reaction volume (109) of a reactor (103); b) heating the active material (104) by means of the joint input of heat from an alternative energy source (108) and heat recovered from radiation losses of the active material (104), reducing the active material (104) and generating oxygen in the reaction volume (109);c) evacuating the oxygen produced through a first evacuation path (11 1 ) from the outlet (106) of the reactor (103); d) injecting water into the reaction volume (109) of the reactor (103), oxidizing the active material (104) and producing hydrogen; and e) evacuating the hydrogen produced through a second evacuation path (112) from the outlet (106) of the device (100), characterized in that step b) of heating the active material (104) is carried out by means of the joint contribution of heat from a non-solar energy source (108) and heat recovered from the radiation losses of the active material (104), where the contributed flow of heat recovered from the radiation losses of the material is up to 99% of the radiation emitted by the active material (104).

2. Method for producing hydrogen according to claim 1, wherein the supply and evacuation of compounds and energy is carried out automatically.

3. The process for producing hydrogen according to any one of claims 1 to 2, wherein the process is carried out in a vacuum.

4. The process for producing hydrogen according to any one of claims 1 to 3, wherein the energy emitted by the hot material is retained within the reaction volume (109) by a heat trap (102).

5. The process for producing hydrogen according to any one of claims 1 to 4, wherein the process comprises a final step of separating the hydrogen and the remaining water evacuated in step e).

6. The process for producing hydrogen according to any one of claims 1 to 5, wherein the active material is in powder form or is a porous material.

7. The process for producing hydrogen according to claim 6, wherein the active material occupies at least 75% of the reaction volume.

8. Device for producing hydrogen by means of thermochemical water dissociation cycles under (quasi-)isothermal conditions, from an active material capable of sustaining redox cycles with hydrogen production in each oxidation half-cycle, wherein said material is partially reduced in the reduction half-cycle, consuming energy in the form of heat, and oxidized in the oxidation half-cycle, due to the interaction with water, characterized in that said device comprises: a. a reactor (103) comprising a reaction volume (109) configured to contain an active material (104) therein and to carry out the thermochemical water dissociation cycles; b. an inlet (105), in connection with a water injection system (110), configured to introduce a flow of water into the reactor (103); g.a gas outlet (106), said outlet comprising a two-way system, wherein a first evacuation path (111) is configured to evacuate produced oxygen from the reaction volume (109), and a second evacuation path (112) is configured to evacuate produced hydrogen and remaining water from the reaction volume (109); c. a non-solar energy source (108) configured to heat, in use, an active material (104) disposed in the reaction volume (109) of the reactor (103); d. a heat trap (102) configured to confine emitted energy in the form of radiation within the reactor and absorb it in an area close to the reaction volume (109) of the reactor (103), comprising at least one radiation reflector. located inside an airtight container (101), configured to reflect, in use, up to 99% of the radiation emitted by a hot active material (104); and e. a process control system (107) comprising a set of valves and pumps configured to regulate the stages of the redox cycle, synchronizing the opening and closing of the water injection system (110) and the output of the generated gases.

9. The device for producing hydrogen according to claim 8, wherein the non-solar energy source (108) is an energy source selected from the group consisting of: a resistive heat source, an inductive heat source, a source based on a high temperature fluid, an electromagnetic radiation source, a nuclear energy source, a geothermal energy source and a combination of these sources.

10. The device for producing hydrogen according to claim 9, wherein the heat source is an electrical resistor arranged around the reaction volume (109). 1 1. The device for producing hydrogen according to claim 9, wherein the heat source is a high temperature fluid contained in a concentric jacket (301) surrounding the reactor (103).

12. The device for producing hydrogen according to claim 9, wherein the heat source is a high temperature fluid contained in a tube, wherein said tube is concentrically surrounded by the reactor (103).

13. The device for producing hydrogen according to claim 9, wherein the source of electromagnetic radiation is a radiation source selected from the group consisting of: microwaves or infrared radiation.

14. The device for producing hydrogen according to any one of claims 8 to 13, wherein the thermal trap (102) is attached to the airtight container (101).

15. The device for producing hydrogen according to any one of claims 8 to 14, wherein the reaction volume (109) is configured to contain active material (104) arranged in a layer adhered to a substrate, either as a micrometric powder or submicron, either within a porous container or as a massive porous material.

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