DE102013211249A1 - Stepwise thermochemical storage of solar heat by redox materials - Google Patents

Abstract

The present invention addresses the task of effectively storing solar thermal energy from concentrating solar systems that work with air as the working medium, in order to convert it later into electricity when the sun has set or is covered by clouds.

Description

In the present invention, it is important to efficiently store the thermal energy collected by a solar tower power plant using air as a heat transfer medium during "on-sun" operation in order to later convert it into electrical energy in clouds or after sunset (" off-sun "operation). On-sun operation here describes the state in which concentrated solar radiation strikes the radiation receiver, while in off-sun operation no concentrated solar radiation is used.

Such power plants currently store the solar energy as sensitive heat in solids and can be operated "off-sun" only about 1.5 hours.

Electrical energy can be obtained from solar radiation in the following way [ IEA Technology Roadmap Concentrating Solar Power, (2010) ]: The solar radiation is focused on a focus by mirrors that track the sun. This provides heat at medium to high temperature. In order to convert the high-energy thermal energy obtained in this way into mechanical work, a heat exchanger (radiation receiver) is used, which is the focus of the concentrated radiation. In the receiver, a heat transfer medium (air, water, thermal oil, sodium or molten salt) is first heated solar. The resulting increased enthalpy of the heat transfer medium is then used to generate steam in order to operate a conventional power plant process (Rankine, Brayton or Stirling process). Direct solar radiation may be concentrated by a number of different solar thermal power plant technologies, e.g. B. parabolic trough, tower and dish systems.

Commercial solar thermal power plants have the potential to meet the world's enormous expectations for clean electrical energy. However, the demand for electrical energy is still high even a few hours after sunset. Therefore, solar thermal power plants must also be able to provide this "after sunset" base load power. One of the major differences between solar thermal power plants and other renewable energy technologies is the ability to integrate thermal storage into solar thermal power plants so that they can provide power even after sunset. The concept of thermal storage is simple: During the day, excess heat is introduced into a storage medium. When power generation is required after sunset, the stored heat is channeled into the steam cycle and the system can continue to produce electricity.

There are three ways to convert thermal energy storage, depending on how the heat is stored: sensible heat storage, latent heat storage and thermochemical heat storage, the latter referring to heat required by a reversible endothermic chemical reaction [ A. Gil, M. Medrano, I. Martorell, A. Lazaro, P. Dolado, B. Zalba, LF Cabeza, State of the art on high temperature thermal energy storage for power generation. Part 1 - Concepts, Materials and Modeling, Renewable and Sustainable Energy Reviews 14, 31-55, (2010) ].

With sensible heat storage, thermal energy can be absorbed by temperature changes from a storage medium. Sensitive thermal storage consists of a storage medium, a container (usually a tank), an inlet and an outlet. During on-sun operation, part of the solar-heated heat transfer medium is used to increase the temperature of the storage medium. During off-sun operation, the cold heat transfer medium flows back through the still hot storage medium and is thus brought to a temperature level sufficient for the operation of the steam cycle. The storage tanks must on the one hand contain the storage medium and on the other hand minimize the heat loss to the environment (provide isolation). Fixed storage media are typically designed as fixed beds, whereby a further heat transfer medium is needed. When this medium is a liquid, the heat capacity of both the solid and the liquid is not negligible and the system is called a "dual storage" system. One advantage of "dual storage" systems is that low cost materials such as stone, sand, high temperature concrete [ R. Tamme, CSP plants, Solar Power Generation Conference, Barcelona, (2009) ] or cast ceramics can be used.

At present only two different storage technologies are used in medium and large solar thermal power plants. The PS10 (10 MW) power plant in Solucar near Seville / Spain uses the steam accumulator concept, meaning that sensible heat is stored in pressurized, liquid water. At full load, part of the steam generated is used to charge the storage system [ Solúcar. PS10: A 11.0 MWe solar tower power plant with saturated steam receiver. Internal report; (2006) ]. It consists of four tanks, which are operated one after the other, depending on the state of charge. The other industrially used, mature storage technology is the liquid salt storage tank with two tanks. This technology is used by solar thermal power plants such as ANDASOL (1-3 50 MW parabolic trough power plants in southern Spain), ARCHIMEDE (parabolic trough power plant) and GEMASOLAR (17 MW Solar tower power plant). Salt melts are stored in two tanks, one hot and one cold [ http://www.estelaolar.eu/index.php?id=32 ]. Salt, which is fed into the hot tank for storage, is previously heated by means of a heat exchanger. When heat is required from the storage tank, the salt is returned through the same heat exchanger, heating an oil to just under 400 ° C.

Both concepts involve several disadvantages, which are described in more detail below. Therefore, there has recently been much interest in alternative storage materials and concepts currently being tested on a laboratory and pilot scale. These include, for example, inexpensive solids as sensitive heat storage or phase change materials for latent heat storage. DLR is currently investigating the efficiency, stability and cost-effectiveness of solid heat storage systems (high-temperature concrete or cast ceramics) for parabolic trough power plants. Such systems use an established heat transfer medium (thermal oil) in the solar field, which is passed in pipes through the solid heat storage to deliver heat to the solid during solar operation. The advantage of this concept is the low cost of the solid.

Sensitive heat accumulators are also the preferred storage technology for solar towers, which use air as the heat transfer medium. An example of this is the solar tower Jülich (STJ), which was put into operation in 2009 ( ). An open volumetric receiver developed by the DLR is used there. Such a receiver is made of silicon carbide honeycomb structures ( ) [ S. Zunft, M. Hänel, M. Krüger, V. Dreißigacker, High-temperature Heat Storage for Air-Cooled Solar Central Receiver Plants: A Design Study, Proceedings of 15th International Solar PACES Concentrating Solar Power Symposium, Berlin, Germany, September 15th -18th, (2009) .].

In its primary cycle, the ambient air is heated at atmospheric pressure up to temperatures of about 700 ° C; however, it may be possible to exceed this temperature in future plants using the same technology.

With this solar heat, a steam generator is operated, which generates steam at 100 bar and 500 ° C and drives a 1.5 MWel turbine-generator combination.

Parallel to the steam generator and receiver, a sensible heat accumulator is integrated into the power plant and implemented as an air-cooled combined regenerator tank in which the air is passed through and transfers its enthalpy to a solid medium (1 (b)).

With this type of storage solution, which is used in various industrial applications, is a gaseous heat transfer medium, eg. Air, in direct contact with a solid storage medium and exchanges heat as it flows along a flow path through the storage medium. The completely realized storage solution consists of a rectangular housing of 7 m × 7 m × 6 m ( ) and is similar to an industrial regenerative burner system [ S. Zunft, M. Hänel, M. Krüger, V. Dreißigacker, F. Göhring, E. Wahl, Jülich Solar Power Tower - Prospects of 15th International SolarPACES Concentrating Solar Power Symposium, Perpignan , France, 21th-24th September, (2010) ].

Instead of a monolithic design, the memory is divided into four equal sized chambers, which operate in parallel and are connected by a dome and connecting tubes. Each chamber is filled with a series of building blocks of ceramic storage material in the form of honeycomb structures ( 1 (d) ) to provide a large heat exchange area between the air and the solid storage medium. The total volume of the inventory is 120 m ³ . The full load discharge duration of this heat accumulator is limited to about 1.5 hours. The storage system is integrated in the solar tower near the receiver and the steam generator. At nominal operating conditions, the storage system is operated between 120 and 680 ° C, ie during solar operation, "hot" air from the solar tower is supplied to the inlet of the storage medium at a temperature of about 680 ° C and leaves at the outlet at the bottom of the storage medium a temperature of about 120 ° C ( 1 (b) ). Thus, the temperature of the storage medium (and thus also the air that flows through it) during the on-sun operation ("charging") can be qualitatively as in 1 (f) with the upper area showing the hotter zones and the lower area showing the colder zones. During off-sun operation, the airflow is reversed: "cold" air is introduced through the lower end of the storage medium to thereby be heated as it flows toward the already "hotter" upper end of the storage before returning to the storage compartment Electricity generating block is introduced. In principle, the technology is applicable to atmospheric systems and pressurized systems.

Alternatively, thermal energy can be stored almost isothermally with latent heat storage material (LHS) by means of a series of substances by the latent heat to a Phase change, such as the heat of fusion in the solid-liquid transition or how the heat of vaporization in the liquid-vapor transition, is used. The overall concept of storage using latent heat is the same as for solid state storage; the difference is that with latent heat storage, the solar heat provided during solar operation is not used to increase the temperature of the heat storage medium, but to melt it; the same amount of heat can be recovered by solidifying the heat storage medium during off-sun operation. For this process to be used, the storage material should have a melting temperature in the range of loading and unloading temperatures of the HTF; Substances used are called Phase Change Material (PCM). Material candidates for latent heat storage are salts (NaNO ₃ / KNO ₃ ) [ D. Laing, R. Tamme, WD Steinmann, PCM storage for process heat applications, 1st Annual Conference of the European Technology Platform on Renewable Heating and Cooling, Bilbao, 23-24 / 02/2010 .].

Thermochemical storage concepts have not yet reached this stage of development. They rely on the use of the heat of reaction of reversible chemical reactions for the "storage" of solar heat. This type of storage requires that the chemical reactions involved be completely reversible. The working principle is as follows: the heat generated by the solar receiver during on-sun operation is used to power an endothermic chemical reaction; if the reaction is completely reversible, the thermal energy can be fully recovered by the reverse reaction that occurs during off-sun operation). The heat is released at a constant temperature defined by the reaction equilibrium. Several reversible reactions with high heat of reaction have been proposed for this application: a category includes only gaseous starting materials and products such. B. ammonia dissociation (reaction 1) [ K. Lovegrove, A. Luzzi, I. Soldiani, H. Kreetz, Developing ammonia based thermochemical energy storage for dish power plants, Solar Energy, 76, 331-337, (2004) ], another category addresses gas-solid decomposition reactions, the most typical of which are the decomposition of metal hydroxides (reaction 2), carbonates (reaction 3) and oxides (reaction 4) [ WE Wentworth and E. Chen, Simple thermal decomposition reactions for storage of solar thermal energy, Solar Energy, 18, 205-214, (1976) ; F. Schaube, A. Wörner, R. Tamme, High-temperature thermo-chemical heat storage for CSP using gas-solid reactions, Proceedings of 16th International SolarPACES Concentrating Solar Power Symposium, Perpignan, France, September 21st-24th, (2010) ]. NH ₃ + ΔH ↔ 1/2 N ₂ + 3 / 2H ₂ (1) ΔH = 49 kJ / mol _react Ca (OH) ₂ + ΔH ↔ CaO + H ₂ O (2) ΔH = 100 kJ / mol _react CaCO ₃ + ΔH ↔ CaO + CO ₂ (3) ΔH = 167 kJ / mol _react Co ₃ O ₄ + ΔH ↔ 3CoO + 1 / 2O ₂ (4) ΔH = 200 kJ / mol _react

As already mentioned, of the proposed gas-solid reactions for the thermochemical storage applications (reactions 2-4 above), the use of reactions of redox pairs of polyvalent solid oxides has several advantages that make them attractive for large-scale use [ B. Wong, L. Brown, F. Schaube, R. Tamme, C. Sattler, Oxide-based thermochemical heat storage, Proceedings of 15th International SolarPACES Concentrating Solar Power Symposium, Perpignan, France, September 21st-24th, (2010) ; General Atomics, Thermochemical Heat Storage for Concentrated Solar Power Based on Multivalent Metal Oxides, CSP Program Review, 18th May, (2011) .].

Several oxide-based thermochemical redox reactions are accompanied by a high degree of heat release and are therefore in principle able to store and release the heat. Potential material candidates in the interesting temperature range are BaO ₂ , Co ₃ O ₄ , Mn ₂ O ₃ , CuO, Fe ₂ O ₃ , and mixed oxides of these systems. The following exemplary reaction schemes are sorted by increasing oxidation / reduction temperature [aa O]: 4MnO ₂ + ΔH 2Mn ₂ O ₃ + O ₂ (5) T = 30 ° C ΔH = 42 kJ / mol _react 2BaO ₂ + ΔH 2BaO + O ₂ (6) T = 690 ° C ΔH = 81 kJ / mol _react 2Co ₃ O ₄ + ΔH 6CoO + O ₂ (7) T = 870 ° C ΔH = 202 kJ / mol _react 6Mn ₂ O ₃ + ΔH 4Mn ₃ O ₄ + O ₂ (8) T = 1000 ° C ΔH = 32 kJ / mol _react 4CuO + ΔH 2Cu ₂ O + O ₂ (9) T = 1030 ° C ΔH = 64 kJ / mol _react

Currently, TCS concepts are being studied in nationally and internationally funded research projects. Among other things, CSP-related research institutions are involved, which are working to demonstrate this concept, at least for special reaction systems, in a pilot plant with corresponding storage units. The concept of coating ceramic, monolithic, porous structures with solid active material -. B. catalysts - to improve high-temperature gas-solid reactions has long been used on an industrial scale. The best example of this is ceramic honeycomb bodies coated with high surface area oxide ("primer") catalyst material, a technique used in three-way diesel particulate filter (DPF) catalysts in any automotive exhaust system , An alternative to honeycomb bodies, before being catalytically coated, are the cellular porous ceramic foam structures used to filter molten metals and flame diffusers. Such typical honeycomb bodies and foams, as well as their respective microstructure are in the shown. shows the net-like structure of foams.

In the same field of catalytic treatment of harmful gases and particles, the diverse conditions - often opposing - chemical reactions, oxidation of CO and hydrocarbons, reduction of NOx and Oammeln and oxidize soot particles, have led to the use of multiple, simultaneously used catalysts, and the Use of synergies between these catalysts. In fact, all modern diesel lean-burn engines are based on a "cascading" principle. Three different porous active substances and converters act in series: (i) oxidation catalysis to remove CO and HCs, (ii) DPF to remove soot particles, and (iii) specific NOx removal systems (de-NOx), such as Lean NOx traps (LNT ) or converter based on selective catalytic reduction (SCR). [ RM Heck and R. J, Farrauto, Catalytic Air Pollution Control Commercial Technology, Van Nostrand Reinhold, New York USA, (1995) ].

Since such porous monolithic structures occupy much space in the vehicle, the current state of the art is to combine these multiple reactions and to avoid adverse effects between the various materials, a sequential or simultaneous use of the various catalytic materials. Furthermore, multidimensional designs are introduced, such. B. multilayer catalyst systems and / or a spatial separation of the catalytic materials over and along the porous structure. Two examples are "separate primers" and "zonal coating". Separate primers have been developed to spatially separate precious metals by applying them to the primer's own metal oxides before being applied to the substrate. Typical examples are multilayer catalyst systems: The catalyst system is spatially subdivided into two or more layers which contain well separated, different dispersions of active metals and their accelerators ( - Schematic example of a separate primer [ M. Berndt, P. Landri, An overview about Engelhard's approach to non-standard environmental catalysis, Catalysis Today 75, 17-22, (2002) ]]. [aa O.]). Alternatively, "zonal coating" involves coatings of different zones on the monolithic substrate with different catalyst shapes. (Scheme in - "zonal" coating of channels of a DPF [ A. Punke, G. Grubert, Y. Li, J. Dettling, T. Neubauer, Catalyzed Soot Filters in Close Coupled Position for Passenger Vehicles, SAE Technical Paper 2006-01-1091 ]). For example, a substrate with two axial zones can replace two separate substrate layers with the corresponding catalysts, with "radial" zones in the middle containing a higher proportion of noble metals, since here the gas flow is usually higher. In all these applications, the ceramic structure is inherently inert with respect to the desired gas-solid reaction: it is used either as a physical filter or as a chemically inert, porous structure on which chemically active species can be dispersed. For this, in the case of chemical reactions, the volume-related product yield is small, since the bulk of the reactor volume is coated with chemically inert material. The volume-related product yield of reactors structured in this way can be achieved, for example, by producing the support structure completely from the active material. The disadvantage here is in most cases significantly higher price of the active material compared to the ceramic (when, for example, catalysts such as platinum are used). Indeed, such an approach has been extruded from the solid material by the SCR with NH ₃ , which requires large surfaces of titanium / vanadium mixtures or zeolitic catalysts, to control NOx emissions from stationary sources [ Lachman, JL Williams, Extruded Monolithic Catalyst Support, Catalysis Today, 14 (2), 317-329, (1992) ].

In the case of honeycomb reactors, an extruded body of Fe ₂ O ₃ , which is a catalyst for the dehydrogenation of ethylbenzene to styrene, reports [ WP Addiego, W. Liu, T. Boger, Iron oxide-based honeycomb catalysts for the dehydrogenation of ethylbenzene to styrene, Catalysis Today, 69, 25-31, (2001). ; WP Addiego and W. Liu, Extruded honeycomb dehydrogenation catalyst and method, US Patent 6,461,995 B1 , (2202)].

Another patent describes the preparation of foams of chromium oxide (Cr ₂ O ₃ ) or cobalt-chromium oxide (Co _0.2 Cr _0.8 O _x ) for testing the production of synthesis gas by means of the partial oxidation of light hydrocarbons [K. Kourtakis, AM Gaffney, L. Wang, Reticulated ceramic foam catalysts for synthesis gas production, US 6,630,078 B2 , (2003)].

In the field of "solar process engineering", especially in thermochemical cycle processes, the use of oxides as redox materials is known:
Based on this established technology, the idea of coating similar ceramics - which are however able to absorb solar radiation - in monolithic structures such as honeycombs and foams with catalytically active material or redox materials has been developed. Indeed, the first publications on these CSP structures included ceramic foams: The first example of a "structured" solar reactor is the "CAESAR" experiment conducted by Sandia National Laboratories (SNL) and the DIR in the solar reforming of methane CO _{2 was} demonstrated [ R. Buck, JF Muir, RE Hogan, RD Skocypec, CO2 reforming of methane in a solar volumetric receiver / reactor: The CAESAR project. Solar Energy Materials, 24, 449-463, (1991) ]. Here, a disk of alpha-alumina mullite foam, coated with gamma-alumina and rhodium catalyst was placed in an absorber that was in the focus of a parabolic mirror.

The second and third generation SiC foam based solar chemical receiver reactors have been designed and tested for the tower of the Weizmann Institute of Science (WIS) Solar Research Center in Israel [ A. Wörner and R. Tamme, CO2 reforming of methane in a solar driven volumetric receiver-reactor, Catalysis Today, 46, 165-174, (1998) ]. Ceramic honeycomb structures, although extensively studied and established as volumetric receivers for solar radiation, became T. Fend, B. Hoffschmidt, R. Pitz-Paal, O. Reutter and P. Rietbrock, Porous materials as open volumetric solar receivers: Experimental determination of thermophysical and heat transfer properties, Energy, 29 (5-6), 823- 833, (2004) ; B. Hoffschmidt, V. Fernández, R. Pitz-Paal, M. Romero, P. Stobbe, and F. Téllez, The development strategy of the HiTREC volumetric receiver technology - up-scaling from 200kWth via 3MWth to 10MWel, in Proceedings of 11th SolarPACES International Symposium on Concentrated Solar Power and Chemical Energy Technologies, Zurich, Switzerland, September 2002 ; CC Agrafiotis, I. Mavroidis, AG Konstandopoulos, B. Hoffschmidt, P. Stobbe, M. Romero, V. Fernandez-Quero, Evaluation of porous silicon carbide monolithic honeycombs as volumetric receivers / collectors of concentrated solar radiation, Journal of Solar Energy Materials and Solar Cells, 91, 474-488, (2007) ], were not used as building blocks for solar reactors before (2005) the HYDROSOL research group recently developed the concept of a monolithic solar reactor with honeycomb structures made of SiC honeycombs (cf. 1 (c) ) coated with redox pairs of iron mixed oxides for the implementation of thermochemical cycle processes for hydrogen production by water splitting with solar energy presented [ C. Agrafiotis, M. Roeb, AG Konstandopoulos, L. Nalbandian, VT Zaspalis, C. Sattler, P. Stobbe, AM Steele, Solar water splitting for monolithic reactor, Solar Energy, 79 (4), 409-421 , (2005) ; M. Roeb, C. Sattler, R. Klüser, N. Monnerie, L. deOliveira, AG Konstandopoulos, C. Agrafiotis, VT Zaspalis, L. Nalbandian P. Stobbe, AM Steele, Solar hydrogen production by a two-step cycle based on mixed iron oxides, Journal of Solar Energy Engineering - Transactions of the ASME, 128, 125-133, (2006) ; http://www.hydrosol-project.org/ ; CC Agrafiotis, C. Pagkoura, S. Lorentzou, M. Kostoglou, AG Konstandopoulos, Hydrogen Production in Solar Reactors, Catalysis Today, 127, 265-277, (2007) ], currently scaled-up and demonstrated to the 100-kW level [ M. Roeb, J.-P. Sack, P. Rietbrock, C. Prahl, H. Schreiber, M. Neises, D. Graf, M. Ebert, W. Reinalter, M. Meyer-Grunfeldt, C. Sattler, A. Lopez, A. Vidal, A. Elsberg, P. Stobbe, D. Jones, A. Steele, S. Lorentzou, C. Pagkoura, A. Zygogianni, C. Agrafiotis, AG Konstandopoulos, Test Operation of a 100-kW pilot plant for solar hydrogen production from water on a solar tower, Solar Energy, 85, 634-644, (2011) ]. Nowadays, in addition to honeycomb structures of Japanese [ N. Gokon, T. Kodama, N. Imaizumi, J. Umeda, T. Seo, Ferrites / zirconiacoated foam device prepared by spin coating for solar demonstration of thermochemical water-splitting, International Journal of Hydrogen Energy, 36 (3), 2014 -2028, (2011) ] and French [ HI Villafán-Vidales, S. Abanades, C. Caliot, H. Romero-Paredes, Heat transfer simulation in a thermochemical solar reactor based on a volumetric porous receiver, Applied Thermal Engineering, 31 (16), 3377-3386, (2011) ] Working groups also used ceramic foams as support structure in solar-supported water splitting reactors.

Based on the above-mentioned activities, recently made reticulated porous ceramic foams consisting entirely of the active material - cerium oxide (CeO ₂ ) in this case. These were tested on a laboratory scale under solar thermal high temperature conditions (1600 ° C) for carbon dioxide cleavage (CO ₂ ) using thermochemical redox cycles [ P. Furler, J. Scheffe, M. Gorbar, L. Moes, U. Vogt, A. Steinfeld, Solar Thermochemical CO2 Splitting Utilizing a Retarded Porous Ceria Redox System, Energy and Fuels, 26 (11), 7051-7059, ( 2012) ].

The two storage technologies that have been commercially implemented in existing large scale solar thermal power plants are the "steam storage" concept and the molten salt. The former is characterized by high investment costs for the high-pressure tank and therefore, from an economic point of view, not suitable for further upscaling. The latter is characterized by a number of disadvantages: protection against freezing requires heating, which increases the parasitic energy consumption. In addition, large amounts of the salt are needed; For example, in the case of the Andasol power plant (7.5 hours of storage), 28000 tonnes will be used [ A. Meffre, J. Lambert, X. Py, R. Olivés, G. Matzen, V. Montouillout, C. Bessada, P. Echegut, N. Calvet, U. Michon, Correlations between structural properties and elaboration conditions for sensible heat International SolarPACES Concentrating Solar Power Symposium, Perpignan, France, September 21st-24th, (2010) ]. Finally, the potential of this technology for higher efficiencies is only marginal due to the maximum temperatures limited to 565 ° C decomposition of molten salt. On the other hand, memory systems based on PCMs can be significantly reduced in size compared to systems that do not use phase transition. However, the design of the heat transfer and the media is more difficult and experience shows that the performance of the materials deteriorates after some solidification melt cycles. The poor thermal conductivity of the solid phase salts is accounted for by manufacturers of salt / graphite blends and by the implementation of heat exchanger concepts such as tubes, finned or "sandwich" configurations [supra]. On the other hand, in the case of the STJ, the storage system, which is based on a sensitive heat storage, is only able to ensure off-sun operation for about 1.5 hours.

Thermochemical heat storage has several important advantages over other storage technologies (latent and sensible heat storage): higher energy storage densities attainable, unlimited storage time near ambient temperature, heat pump capability and suitability for large scale installations. Thermo-thermal thermo-thermal pathways allow systems with inherent energy storage for continuous (24 h) electricity generation, an issue that is becoming increasingly important as the world moves toward a renewable energy-based economy.

However, among the various reaction systems proposed for TCS, the NH3 / N2 system has the inherent disadvantage of requiring high pressures, making long-term storage of its gaseous constituents impractical. In the TCS applications proposed for gas-solid reactions, the decomposition of the hydroxides and carbonates, as in reactions (2) and (3) above, involves the production of gaseous products such as H ₂ O or CO ₂ , which condenses for storage or have to be compressed, resulting in parasitic energy losses. Moreover, the major engineering challenge in such gas-solid reactions is to appropriately develop and operate TCS reactors that function as heat exchangers at the same time and efficiently. Fixed bed reactor concepts, heretofore proposed especially for the CaO system, involve either indirect heat transfer in the reaction zone through an external heat exchanger and heat transfer through the storage reactor wall or direct heat transfer by means of flow of a heat transfer fluid through the reaction zone which simultaneously contains the gaseous reactant and / or catalyst Could be inert gas. On the other hand, fluidized bed reactor designs, while capable of improving mixing, diffusion and heat transfer properties, require high gas pressures and transport / recirculation (hot) particulate matter and, in addition to the expected problems of attrition and corrosion / erosion, cause high parasitic energy consumption and economic problems. Similar problems associated with the loading and unloading of powder in storage tanks with moving parts and complicated mechanical parts concern other proposed storage concepts, such as solar towers, which limit the potential for efficient upscaling.

In the field of solar energy storage, redox reactions of the oxides of metals with multivalent states (such as reactions (5) - (9) above) have been discussed since 1978 for TCS [ RG Bowery and J. Justen, Energy Storage using the reversible oxidation of barium oxide, Solar Energy, 21, 523-525, (1978) ; MA Fahim and JD Ford, Energy storage using the BaO2-BaO reaction cycle, The Chemical Engineering Journal, 27, 21-28, (1983) ; ER Stobbe, BA de Boer, JW Geus, The Reduction and Oxidation Behavior of Manganese Oxides, Catalysis Today, 47, 161-167, (1999) ; H. Matsuda T. Ishizu, SK Lee, M Hasatani, Kinetic study of Ca (OH) 2 / CaO reversible thermochemical reaction for thermal energy storage by means of chemical reaction, Kagaku Kogaku Ronbunshu, 11, 542 (1985) ; KN Hutchings, M. Wilson, PA Larsen, RA Cutler, Kinetic and Thermodynamic Considerations for Oxygen Absorption / Desorption Using Cobalt Oxide, Solid State Ionics, 177, 45-51, (2006) ]; however, recent systematic comparative studies of such systems have appeared in the literature [loc. cit.], which are limited to single oxide systems and fixed bed reactors in the laboratory.

On the other hand, as already mentioned, for the storage of sensible heat currently porous monolithic structures are used in direct contact with solar heated air flowing through their channels (during the "charging"). This process is purely thermal; no chemical reactions take place [loc. cit.].

Similar ideas have recently been published with respect to thermal latent heat storage in publicly available literature. The application of active heat storage materials such as salts as phase change material to honeycomb or foam structures by impregnation is suggested so as to produce a composite material which can store and release heat [a. a. O.].

With the same concept of latent heat storage using phase change materials, the idea of combining cascades of materials of different chemical composition in a suitable manner in space has been proposed and implemented in terms of their thermochemical properties. The so-called "Cascaded Latent Heat Storage" (CLHS) has been proposed and verified experimentally and numerically with a cascade of three phase change materials, in which a mixture of salts in the solid state with an increasing melting temperature of the lower operating temperature of the memory module are cascaded to the maximum operating temperature according to the various properties of the medium for transmitting sensible heat and the media for latent heat storage. [ F. Dinter, M. Geyer, R. Tamme, Thermal energy storage for commercial applications (TESCA), a feasibility study on economic storage systems. Springer-Verlag, Berlin, (1991) ; H. Michels, R. Pitz-Paal, Cascaded latent heat storage for parabolic trough solar power plants, Solar Energy, 81 829-837, (2007) ].

In this connection, the present invention proposes to use the "thermo chemical storage" (TCS) of solar heat together with the storage of sensible heat, i. H. utilizing the thermal effect of reversible chemical reactions: solar heat generated during on-sun operation that increases the enthalpy of the heat transfer medium is used to cause an endothermic chemical reaction of a particular compound; if this reaction is completely reversible, the total thermal energy can be recovered by the back reaction occurring during off-sun operation - in this concept, the chemical compound itself is the so-called heat storage medium (HSM). Among the various reversible chemical reactions which can be used for this purpose, the invention proposes to use two gas-solid redox reactions between oxygen (from ambient air) and multivalent solid-state oxides: in particular the thermal reduction of the oxide in the oxidized state whereby heat is converted into chemical bonds (heat storage) and oxidation in air of the oxide in the reduced state whereby heat is released (heat recovery). In such reactions, air can be used simultaneously as a heat transfer fluid and as a reactant; therefore, such systems can be ideally coupled with CSP solar tower systems that use volumetric receivers and air as the heat transfer medium.

The specific technical application of the invention is the achievement of more efficient heat transfer between the solar heated heat transfer fluid and the heat storage medium (the oxide) and thereby extending the operation of the plant without sun over the current state of the art.

In this regard, the novel concept presented in the invention includes cascades of structures as a heat storage medium with different redox materials arranged in a certain rational pattern in space according to their thermochemical properties and the local temperature of the heat transfer medium.

In particular, the invention relates to the meaningful combination of thermochemical properties and the spatial distribution of redox materials that can be thermally oxidized and reduced under air atmosphere at temperatures of preferably 100 ° C to 1200 ° C, which can be in integrated chemical reactors / heat exchangers for the thermochemical storage of solar heat in commercial solar power plants using air as the heat carrier can be used. This cascading increases and maximizes the utilization of the heat transfer fluids and the storage of its enthalpy.

Individual stages of the cascade can be used as the redox system instead of polyvalent solid oxides, and metal oxide / metal systems in reduced form do not consist of the metal having the lower valence but the metal itself (see below).

Such a cascaded arrangement is in principle compatible and, in combination with all possible heat exchanger / reactor designs, with the particles of each oxide material either randomly distributed in space or used in a particular manner at the reactor level, in an improved heat exchange manner the solid storage medium and the gas-heat transfer fluid with high volumetric heat storage capacity and acceptable pressure drop to combine. In this regard, the cascade of individual stages may be either a fixed bed or porous structures such as honeycombs or foams either coated with a redox material or entirely from any combination of the above materials.

The invention may be combined with a variation of porosity within the single cascade stage (i.e., not only on but also across the reactor cross-section). For example, in relation to the earlier subclaim, honeycomb or foam structures having different porosity may be distributed over a particular reactor / heat exchanger stage cross section to homogenize the velocity profile of the air and thus achieve more uniform heat transfer throughout the reactor.

The cascaded structure may additionally be coupled with various other (non-thermochemical) modes of heat storage, i. H. with storage of sensible or latent heat, meaningful or combined in another configuration, for example, only thermochemical, sensible or latent heat storage.

In summary, the invention can be visualized as a modular, cascaded storage, as a thermochemical reactor / heat exchanger with spatial variation of the functional materials and structural features in three dimensions.

The present invention is derived from all the above developments. The first approach proposes the use of cascaded structures of redox oxides of different chemical composition, in a three-dimensional variation in the space for the thermochemical storage of solar heat, where the generated heat rather than the chemical product per se is of primary interest. So too is the idea of the possible combination with different exchanger-structure concepts, such as the coating or the production of completely porous support structures with / from redox oxides, each not in an integrated reactor / heat exchanger concept for maximizing the volumetric Yield of desired chemical products but to be used for heat storage. In this regard, the present invention differs from the above concepts in the following aspects:
The desired material compositions are not characterized by a high chemical affinity for the production of a particular chemical end product (eg the functionality for water splitting or carbon dioxide cleavage) but by moderate to high heat exposure from oxidation / reduction and high Extent of reversibility marked.

The ambient air is also different: in thermochemical cycles involving thermal reduction and oxidation of the compounds by steam or carbon dioxide (H ₂ O / CO ₂ fission), the thermal reduction is favored only by the partial pressure of oxygen above of the material is very low, ie under an inert atmosphere. In contrast, thermochemical storage takes place under air, so these are materials that can be thermally reduced and oxidized under air atmosphere.

Operating temperatures are also different: in the H ₂ O / CO ₂ fission cases, the reactors are heated directly from concentrated solar radiation to temperatures above 1400 ° C for thermal reduction, while TCS oxide materials operate in a temperature range of 100 ° C C are up to 1200 ° C.

The proposed concept is quite different, not only in terms of structured catalytic reactors as described above, but also of "conventional" heat exchangers / reactors (HEX), which combine a reactor and a heat exchanger in one unit with the incentive to supply / heat removal to improve the endothermic / exothermic reactions, almost as fast as they are absorbed or generated by the reaction [ Z. Anxionnaz, M. Cabassud, C. Gourdon, P. Tochon, Heat Exchangers / Reactors (HEX Reactors): Concepts, Technologies: State-of-the-art, Chemical Engineering and Processing: Process Intensification, 47 (12) 2029- 2050 (2008) ]. On the contrary, for the particular case of TCS, the proposed structure must be optimized to make the heat transfer and the delivery / release between the reactive solid and the flowing gas controllable, permanently for a longer period of time.

The thermochemical storage operating conditions are much milder than those for the solar-thermochemical reactions for water or CO ₂ cleavage; therefore, various problems in the latter applications with materials, stability / volatility at elevated temperatures, thermal stresses during operation, undesirable chemical reactions, etc., are much lower and can be solved much more effectively. The use of cascaded structures of different chemical compositions and possibly a variety of structural characteristic designs that maximize volumetric heat input in a heat exchange module, as proposed in the present invention, enables a compact, effective, modular and scalable integrated heat exchanger. thermochemical reactor Configuration for the storage of solar heat with extended operating time than previously proposed.

The presented invention is aimed at solar thermal power plants (STKW), in which air is used as heat transfer fluid, with the aim of significantly increasing the off-sun discharge time compared to current durations. For this purpose, a transition from the current status, the use of sensible heat, to a thermochemical storage is suggested - as explained below, a closer look at a "sensitive thermochemical" hybrid storage can be made. The invention aims to bridge the "technological gap" of development and application of efficient thermochemical storage reactor / heat exchangers. The new concept provides for the use of cascaded structures as heat storage media, wherein the structures contain different redox oxides and in a certain way, according to their thermochemical characteristics and the local temperature of the heat transfer fluid, are arranged in space. This has the following advantages over other concepts for storing solar heat and the heat storage technologies used in such STCs: The use of thermochemical storage in addition to the common storage of sensible heat and the associated in-situ storage and generation of heat by chemical reactions in addition to the sensible heat results in a higher volumetric storage capacity within a given volume of memory modules.

Among the possible chemical reactions with significant thermal effects is the selection and use of chemical reactions of polyvalent redox oxides with air, which eliminates the need for gaseous reaction products of non-oxide solids with air, for example water vapor from the decomposition of hydroxides or carbon dioxide from the decomposition of carbonates, to be separated from the air stream.

The concept also avoids the use of a built-in heat exchanger. Instead, integrated, air-driven chemical reactor / heat exchanger units can be used. The concept is ideally suited for coupling with STKW, which use open-volumetric ceramic honeycomb cell technologies, since the same heat transfer medium (air) uses, has a simple structure and can be adapted by an appropriate choice of storage materials to the operating temperatures of the power plant. Since there are many theoretically useful oxide materials and thus many thermochemical processes are available, each being conducted at its preferred temperature, the temperature bandwidth is very large. This makes it possible in principle to use a cascade of selected combinations of materials, which is adapted to the temperatures and operating conditions of the solar panels and the power plant block.

This cascaded redox material configuration allows full utilization of the heat output of the heat transfer medium over a wide temperature range.

The cascaded configuration is independent of the type of reactor / heat exchanger selected and thus can be combined with any type of reactor / heat exchanger - beds, honeycomb and foam based systems as well as combinations thereof.

The combination of a cascaded redox oxide-based thermochemical storage concept with a reactor / heat exchanger design that incorporates a large amount of redox material per volume can significantly improve volumetric heat yield.

When the cascaded oxide concept is combined with a suitable heat exchanger design, gas phase to solid phase contact is maximized, thereby bypassing diffusion limitations. This improves the heat transfer characteristic and increases the heat transfer, transport and improves thermal properties and heat recovery properties between the solid storage medium and the gaseous heat transfer fluid.

The cascaded oxide concept is ideally combined with a modular design. The chemical composition of the oxides can be adapted to the individual reactor / heat exchanger modules, so as to obtain high volumetric heat storage capacities at the same time acceptable pressure loss and thus to maximize the overall efficiency.

This modularity of construction can allow a variation of porosities, not only along but also across the reactor cross-section, to homogenize and even out the radial velocity distribution of the heat transfer medium. The overall result of these individual aspects is the possibility of significantly increasing the duration of off-sun operation (discharge) of STKW compared to the currently achieved duration.

In summary, the proposed concept of a redox oxide-based, cascaded, integrated reactor / heat exchanger storage unit that combines the chemical reaction and the heat transfer zone in one connecting a single element, is an elegant, compact and effective technical solution that can overcome many of the drawbacks and limitations of current systems and thus has the best chance of being used in large-scale facilities.

A major advantage of using multivalent solid oxide redox couples as a thermochemical system among the proposed gas-solid reactions for thermochemical storage applications (see reactions 2-4 above) is that air is used both as a heat transfer medium and as a reactant in these reactions can. This can be avoided that CO _{2 must be} compressed / stored or water condensed / vaporized, as is the case with other thermochemical storage. In addition, no additional heat exchanger is needed. Therefore, such a system can be optimally coupled to CSP tower power plants that provide volumetric receivers and air at 650-900 ° C (or higher) as a heat transfer medium.

Based on what has been described above, the starting point of the present invention is to coat the currently used non-reactive honeycomb oxide oxide memory modules with a layer of redox oxide - especially in the oxidized state - for example one of those which is on the left side of the reactions (5 9), which are capable of being thermally reduced and oxidized within the temperature range of the air provided by the tower power plant. For example, in the case of the temperature range of the STJ (680-120 ° C), MnO _{2 would be} appropriate under the above reactions (5-9). However, these temperature ranges are exemplary only and refer to the current operation of this particular solar tower power plant.

Future solar power plants with the same volumetric receiver technology will be able to provide air at higher temperatures, which would allow further oxides of reactions (5-9), such as BaO ₂ , Co ₃ O ₄ , etc., to be used.

Future solar power plants with the same open volumetric receiver technology can provide hot air at even higher temperatures, allowing other oxides from the above reactions (5-9), such as BaO ₂ , Co ₃ O ₄ , etc. also to be used. This concept does not increase the volume of the reactor whereas, in general, a thin coating with respect to the honeycomb walls does not induce any further significant increase in pressure drop. In this configuration, the solar heat is used for the thermal dissociation of the redox material coating and finally stored as chemical energy in the "reduced" state of the respective oxide (eg Mn ₂ O ₃ reaction (5)) instead of just as sensible heat in one chemically inert solid medium (carrier) to be stored. In this regard, the currently existing heat exchanger is converted to an integrated, compact, chemical reactor / heat exchanger.

A schematic representation of the two-stage reaction concept between an oxide particle and an air-gas stream is shown in FIG respectively. 3 (b) each shown for the exemplary case of cobalt oxide. Schematic representation of the thermochemical storage process at the (porous) particle level for the exemplary case of cobalt oxide: (a) thermal reduction (endothermic) of the oxidized form of an oxide particle under the action of hot air, (b) oxidation (exothermic) from the reduced form of the oxide particle in the oxidized state with simultaneous generation of heat, which is used to increase the temperature of the air stream.

If this concept is implemented in an open volumetric air receiver CSP system [aa O.], during on-sun operation the part of the "hot" air flow directed to the memory block ( , above), used to heat the oxidized state of the redox material system (in this case Co ₃ O ₄ ) to a sufficiently high temperature to produce an endothermic "dissociation" to the reduced form (CoO) and gaseous oxygen which is easily mixed with the air stream ( Reaction 7) is entrained and leaves the storage system at lower temperature. Most of the "hot" airflow is supplied to the steam turbine plant to generate hot steam, which in turn is used to generate electricity; The airflow is thus also cooled and mixed with the cool air flow from the storage system, passed back to the solar receiver to be reheated. During off-sun operation ( , below), the "cool air stream" coming from the steam cycle is used to "oxidize" the reduced form of metal oxide in the storage block (ie, based on CoO - reverse reaction 7). Because this reaction is exothermic, it is favored at low to moderate temperatures. Even if the temperature of the air stream is not sufficient to initiate the oxidation reaction, such a reaction can be easily and locally initiated in some other way. Once the oxidation reaction has started, the oxidation reaction is highly exothermic and self-propagates; the enthalpy (heat release) from such a reaction is transferred to the "cool air stream". Thus, it is heated to temperatures high enough to allow the steam turbine cycle to produce off-sun electricity. As already mentioned, is a necessary Requirement that the reversible oxidation-reduction reactions of such systems occur in temperature ranges that can be realized by air streams from the CSP power plant.

Among the various methods of storing heat energy described above, phase change materials (PCCs) and redox oxide thermochemical storage (TCS) latent heat storage systems (LHSs) share the common feature that they are isothermal, i. H. either at the temperature of the melting / solidification of the PCM or at the redox (reduction / oxidation) temperature of the redox oxide material. In order to use the full potential of the two methods, the total mass / volume of the storage medium should be above or below this constant temperature (during on- or off-sun operation). In other words, throughout the duration of the on-sun operation, the heat transferred from the heat transfer fluid to the storage medium must not only be sufficient to melt or reduce the total mass of the storage material, but also the entire Keep mass of the storage material at a temperature above its melting point or the redox temperature; if the temperature falls below the melting point or the redox temperature of the heat storage medium in a spatial area or after some time, solidification or oxidation will occur and this area of the storage material will become unusable.

Therefore, a common problem of both routes could arise if, during on-sun operation of the heat transfer medium, e.g. B. a hot air flow at a certain constant temperature from the solar heat source, heat for melting or thermal reduction supplies. Since the PCM or the redox oxide system continuously absorbs heat, at a certain time the temperature of the heat transfer fluid in the outlet of the storage module falls below the melting or the reduction temperature and the two processes will not continue. This could also happen due to local conditions (heat losses, etc.) at a different location along the mass of the storage material. To solve this problem, in the case of LHS, the so-called "cascaded latent heat storage" (CLHS) concept was proposed. The idea is to combine PCM salts or mixtures of salts with increasing melting temperature. The PCMs should be connected in series from lower to higher operating temperatures according to their melting temperatures in order to match the different characteristics of the transfer medium and the latent heat storage. Particularly for parabolic trough power plants, inter alia, a combination of 5 (five) salts in a row, as shown in Figure 5a, has been discussed. The salt with the highest melting temperature - in the specific example a eutectic mixture of MgCl ₂ / KCl / NaCl - has been selected for the "hot end" of the CLHS (storage above) and the salt with the lowest - NaNO ₃ in the specific case - for the "cold end" (memory below). The idea is conceptually simple: during "on-sun" operation, the "hot" heat transfer fluid at a temperature higher than the melting point of the MgCl ₂ / KCl / NaCl mixture is directed into the store to provide the necessary heat of fusion. Finally, even if the heat transfer fluid comes from the MgCl ₂ / KCl / NaCl "module" of the reservoir at a lower temperature, this temperature is sufficient to provide the heat of fusion of the second material of the cascade and so on. In this way, a better utilization of the possible phase changes and a more uniform outlet temperature is achieved. The concept was confirmed experimentally and numerically in a cascade of three PCMs, namely PCM 1 = KNO ₃ , PCM 2 = KNO ₃ / KCl, and PCM 3 = NaNO ₃ , where the positive effects of a CLHS compared to a non-series LHS have been confirmed [aa O.]. Schematic representation of the principle of operation cascaded latent heat storage (CLHS) with five phase change materials according to [aa O.], (b) adaptation of this idea in the case of thermochemical heat storage for on-sun operation: as an exemplary cascaded thermochemical storage (CTC) concept, which This concept can be applied to fixed bed, honeycomb, foam reactors, as well as in any combination of the above (exemplary applications in shown schematically).

Honeycombs and foams can either be coated with or completely made from the redox material.

With the current approach of storing sensible heat (approach in STJ), the temperature distribution within the storage medium is not uniform, as in qualitatively outlined: The heat transfer medium air enters the storage module at a temperature of 680 ° C and at a temperature of 120 ° C again. If the entire honeycomb cascade were coated with a single oxide, TCS would no longer drain in those zones where the temperature has dropped below the required reduction or oxidation temperature.

Different oxide systems can with increasing reduction / oxidation temperature of cascaded from the "cold" to the "hot end". An exemplary schematic representation of the operating concept with three of the four oxides from reactions (5) - (9) is given in shown. The air coming out of the first module of the cascade is cooled, but nevertheless the temperature is above the necessary redox temperature of the material of the second cascade and thus sufficient to dissociate it to its reduced form and so on. In this regard, although the heat transfer medium is becoming cooler, the enthalpy of the heat transfer medium can be completely stored in the respective "discretized" part of the cascade. The redox systems that can be used in such a cascade include, in addition to polyvalent solid oxides, metal oxide / metal systems whose reduced form does not consist of an oxide but of a metal of lower valency, not as in the reactions. 9, but that of the metal itself according to the reaction scheme (10). This depends on whether such a metal oxide / metal system exists, which can be reduced to the metallic state under air atmosphere within the interesting temperature range (100-1200 ° C) and has a significant and useful decomposition or oxidation heat of reaction: 2MeO + ΔH → 2Me + O ₂ (10) T = 100-1200 ° C; ΔH >> 0 kJ / mol _react

It should be emphasized at this point that, as shown in the same figure, the inventive concept of the cascaded thermochemical store (CTC) proposed herein can be chosen independently of the reactor / heat exchanger design and just as well to all possible ones Reactor / heat exchanger concepts based on a fixed bed or on honeycomb or foam, as well as any combination of them can be applied.

The relevant advantages and disadvantages of any type of heat exchanger concept (ie fixed beds, honeycombs, foams) are widely described in the freely available literature and are known to those skilled in the field of heat exchangers or reactors or ceramic production. In fixed bed reactors / heat exchangers, heat transfer and reaction rates are enhanced by small particle sizes that produce a higher pressure drop. The low thermal conductivity of solid particles is also a problem, as is abrasion. Indeed, the highly porous ceramic structures (such as monolithic honeycombs and foams) offer the inherent advantages over fixed beds: thin walls, high geometric surface area, good gas-solid contact, high gas flow rate with low pressure drop, easy product separation and deposition. If they have the necessary special material properties such as thermal shock resistance and mechanical strength, they are an attractive alternative to fixed beds. When dealing with gas-solid reactions at high temperatures, these "structured" catalytic systems are fully established and the first choice in a variety of catalytic applications, the most notable examples being automotive exhaust gas treatment and catalytic combustion [ A. Cybulski and JA Moulijn (Eds.), Structured Catalysts and Reactors, Taylor and Francis (2005) ]. Typically, such structured catalysts are provided with high void fraction ranging from 0.7 to more than 0.9 (in the case of foams), compared to about 0.5 in packed beds. Scaling of structured reactors is much easier than of random and chaotic systems, such as packed beds, stirred reactors, and fluidized bed reactors. In particular, ceramic foams show an extremely high porosity, with a considerable degree of crosslinking. The result is a low pressure loss. High convection in the tortuous megapores results in improved mass and heat transport. Furthermore, in contrast to their counterparts, the monolithic honeycomb bodies, foams have a considerable degree of radial mixing, which is an advantage in processes which are limited by heat transfer and, moreover, the flow distribution can be adjusted [ MV Twigg and JT Richardson, Fundamentals and Applications of Structured Ceramic Foam Catalysts, Industrial Engineering Chemistry Research, 46, 4166-4177, (2007) ].

In the particular case of thermochemical reactions in a gas-solid system, one approach to maximizing volumetric heat yield is to employ a fixed bed made entirely of the redox material, rather than a honeycomb structure coated solely with the redox material. In this concept, the volumetric yield of thermochemical heat is greatly increased because the entire storage mass participates in the chemical reaction, unlike the coated honeycomb bodies, where only the thin coating is involved in absorption and thermochemical heat generation. On the other hand, such fixed beds are known for a high pressure drop, which leads to a high parasitic consumption. An alternative solution for achieving high volumetric heat yield combined with low pressure loss is the manufacture of monolithic "structured" porous reactors / heat exchangers which cause much less pressure loss than honeycombs or foams made entirely of the redox material , z. B. from an oxide which can be thermochemically reduced and oxidized. A good approach to having a satisfactory performance improved thermo-mechanical volumetric heat yield, as previously described, is a chemically inert porous ceramic support structure such as a honeycomb or foam coated with the redox material. Such inert porous supports are already commercially available products based on materials considered to be manufacturable. In such a case, the concept is similar to the idea of "zonal coating" of a ceramic honeycomb structure in automobile exhaust systems after a treatment already described above.

The modularity of the construction supports the implementation of a combination of the three concepts mentioned above in the same cascade. A particular material characterized by high thermal effects, such as Co ₃ O ₄ , which can be used theoretically, but on the other hand can not be manufactured as rigid, stable and structured shapes such as honeycomb structures or foams due to technical challenges, may be in the form of a Fixed bed can be used. Properly designed can thus a high volumetric heat storage capacity with acceptable pressure drop (schematic example in ) can be achieved.

The modularity of the structure allows many degrees of freedom in terms of design. The smaller modules of the honeycomb structures or of the foams can be produced from different oxides with the same respective extrusion or replication technique and in each case identical tools - only the sintering temperatures will be different. They can be arranged in a suitable sequence on a large scale.

Fixed beds with small-scale bodies of redox material such as small balls, calipers, etc. can be used instead of fine-grained powders. In fact, for fixed-bed heat storage - although it is used in some applications such as flue gas cleaning with regenerative burners - there is no large-scale industrial analogue. In addition, the modularity of the structure offers the possibility of varying the porosity not only in the direction of flow of the fluid but also perpendicular to the flow direction so that the velocity profiles of the fluid are homogenized and smoothed. For example, honeycomb structures with different channel density (channels per square inch-cpsi) or foams with different porosity (pores per inch-ppi) can be distributed across the reactor cross-section (e.g., higher porosity at the reactor rim and smaller reactor center porosity for a homogeneous velocity profile to achieve the air flow in the reactor). In any case, this concept can ideally be linked to the construction principle of modularity, as mentioned above.

Another advantage of this construction principle is that different (also non-thermochemical) types of solar heat accumulators can be combined within the stepwise sequence. In fact, it is apparent that individual stages of the cascade can be populated with phase change materials or inert materials that store only latent or sensible heat. This allows a much wider range of materials to be used and selected to ideally match the temperature profile of the storage system.

In summary, from a general point of view, the entire incremental storage sequence composed of various redox materials can be illustrated as a giant reactor or heat exchanger with 3-dimensional spatial differentiation of functional materials and structural properties. The modular construction principle is illustrated schematically in Figure 6 (a) for reactor or heat exchanger concepts with unstructured materials (fixed bed) and Figure 6 (b) with structured materials (honeycomb structures or foams).

In the latter case, the structures may either be made entirely of the redox material or be coated with an inert base body with the same material. Schematic representation of the modularity of the construction of a complete thermochemical storage unit, adapted to (a) on the basis of cascaded fixed beds, (b) on the basis of small cascaded porous structures, honeycombs or foams which may either be coated with or made of redox oxides and are composed in the total modulus.

The most important application of the invention is to improve the performance, flexibility and economy of power generation by means of CSP and in particular by means of solar tower systems using air as the heat transfer medium. The first large-scale plant of this type, the experimental and demonstration power plant Solarturm Jülich, was put into operation in 2005 and has already reached a state in which it is operated partly commercially.

The importance of an efficient thermal storage system for the realization of the further technical and commercial development of such systems should not be underestimated. CSP systemically has the ability to store heat for later use of electricity generation in the short term.

CSP systems with large storage capacities are capable of producing solar power for basic load supply day and night. It will For emission-free CSP plants, it is possible to compete with coal-fired power plants that have enormous CO ₂ emissions. A current industry focus is to significantly increase the operating temperature of CSP plants to increase overall efficiency and reduce storage costs. Improved heat storage would help to secure production capacity and expand production. It would even allow solar base load power plants.

The invention, which aims to extend the duration of operation of such installations without sun beyond the present limits, is fully in line with the above objectives. Their realization would have a huge impact on the further development and marketing of such facilities. The invention brings some innovative approaches into play, which address both the materials used and the design concept for the technical realization. The use of redox materials for energy storage brings with it a number of salient advantages over the current "storage standard" in the event that some technological hurdles are overcome in further development.

The proposed innovations and associated fallback options aim to eliminate the remaining weaknesses of the technology so that the concept can be brought into line with scaling up and industrial implementation in the short to medium term.

The proposed entirely modular concept of a staged heat accumulator is fully suitable for future scaling up of the technology for industrial application. In fact, the exploitation of the redox properties of the mixed oxides combined with the ability to form them into robust, compact and efficient heat exchanger structures, combined with the possibility of complementing them with other complementary modules of latent heat storage and storage of sensible heat, can be realized a solution for highly efficient storage of solar heat. A drastic cost reduction is to be expected for the memory, which results mainly from the compactness and the efficiency of the proposed storage concept.

The proposed technology represents an added value for all solar tower systems, because a small increase in investment costs enables effective storage of solar energy. This solves the problem of compliance of solar energy supply and electricity demand and paves the way for fully flexible generation of electricity from solar energy. The concept of the present invention of utilizing a staged arrangement of successive oxide materials and placing them in the flow path of the heat transfer medium may also find commercial applications in other industrial processes where such functional materials are inexpensive ceramics such as transition metal oxides. Such applications may involve redox reactions at relatively high temperatures where oxygen transfer takes place.

A definite application example is the Chemical Looping Combustion (the combustion of fuels with inherent separation of CO ₂ and N ₂ with the use of oxidic redox materials) [ J. Adanez, A. Abad, F. Garcia-Labiano, P. Gayan, LF de Diego, Progress in Chemical-Looping Combustion and Reforming Technologies, Progress in Energy and Combustion Science, 38, 215-282, (2012) ]. Not only is the temperature range consistent with that relevant to redox material-based thermochemical storage, but also, as in this application, transition metal oxides are used as oxygen transferring materials.

A possible combination of the staged arrangement with specific structural concepts to increase the volume-related storage density and oxygen yield achieved by exploiting the largest possible amount of oxygen within a given reactor volume becomes even more advantageous when the redox materials are formed into suitably structured objects , The challenge is that these objects must participate in chemical reactions while maintaining their structure and mechanical stability as much as possible.

Most problems in implementing this approach on a large scale result from the potential incompatibility between the nature of the constituents of the storage materials and the tools and methods required to fabricate chemically active and mechanically stable 3-dimensional structures from these materials.

This difficulty can be overcome by the application of the modular construction principle, which makes it possible to manufacture large-scale systems from small elementary units of pore bodies and to arrange them together in groups in a housing, so that mechanical and thermal stresses are minimized. This results in an intelligent design that is easy to assemble and maintain and, in the case of material failure or material degradation, makes it easy to disassemble and replace individual modules independently of the other modules To significantly influence the overall system. Further applications may be - as already mentioned above - processes for the solar-thermochemical production of solar fuels at high temperatures, provided that it is possible to develop cost-effective oxides that are capable of thermochemically cleaving water and / or CO ₂ and the can be reduced in air.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

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Claims (8)

Method for thermochemical storage and provision of energy by means of reversible reactions characterized in that at least two different reactive substances are placed at different positions in a flow of heat transfer fluid and thus form a cascade in which at least one reversible endothermic chemical reaction proceeds at a higher temperature a second reversible endothermic reaction utilizing the heat carrier stream cooled by the first reaction, and wherein the lower energy reaction used for the energy supply of said second chemical reaction proceeds at a lower temperature than the reverse reaction of the first chemical reaction by exiting from the second reverse reaction preheated heat transfer stream uses. A method according to claim 1, characterized in that the reactive substances from the group of multivalent oxides and / or reversibly oxidizable and reducible metals, carbonates and / or hydroxides are selected A method according to any one of claims 1 and 2, characterized in that the second reaction from the group of reversible chemical reactions or the phase change of a material is selected, wherein the reaction proceeds at a lower temperature than the first. Method according to one of claims 1 to 3, characterized in that the reactants are in direct or indirect contact with the heat transfer medium. Method according to one of claims 1 to 4, characterized in that said substances are used in powder form, as a fixed bed, as a foam structure, as a honeycomb structure, as a brick or as a combination of the aforementioned possibilities. A method according to any one of claims 1 to 5, characterized in that the porosity of one stage of the cascade is homogeneous within the stage or varies along the stage or across the stage. Method according to one of claims 1 to 6, characterized in that the heat transfer medium contains air and / or oxygen. A process according to any one of claims 1 to 7, characterized in that at least one or up to all reactions occur at temperatures in the range from 100 ° C to 1200 ° C.

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