Final Report Summary - STOLARFOAM (Thermochemical Storage of Solar Heat via Advanced Reactors/Heat exchangers based on Ceramic Foams)
The main goal of the project was to introduce and demonstrate the operation of an entirely new concept for solar heat storage. ThermoChemical Storage (TCS) exploits the heat effects of reversible chemical reactions: the heat produced by a solar receiver during on-sun operation is employed to power an endothermic chemical reaction; the “consumed” thermal energy can be recovered completely by the reverse, exothermic reaction taking place during off-sun operation. However, the current state-of-the-art solar heat storage concept in air-operated Solar Tower Power Plants is to store the solar energy provided during on-sun operation as sensible heat in porous solid ceramic materials like honeycombs that operate as recuperators during off-sun operation. This storage concept can be rendered from “purely” sensible to “hybrid” sensible/thermochemical one, via coating the chemically inert porous heat exchange modules with oxides of multivalent metals for which their reduction/oxidation reactions are accompanied by significant heat effects (e.g. Co3O4/CoO, BaO2/BaO, Mn2O3/Mn3O4, CuO/Cu2O), or by manufacturing them entirely of such oxides. In this “hybrid” concept, solar-heated air produced during on-sun operation is used, in addition to sensibly heat a porous material, to power the endothermic reduction of the oxide with the higher metal valence state to that with the lower; this thermal energy can be entirely recovered by the reverse, exothermic, oxidation reaction taking place during off-sun operation.
Addressing the major technical challenge for TCS applications which is the proper design and operation of TCS reactors that will operate simultaneously and efficiently as heat exchangers, the project has set forth and implemented for the first time the use of highly porous structures – ceramic foams made partially or entirely from redox oxide materials – as the medium within which the reaction and the heat exchange will take place. The proposed concept combines the demonstrated capabilities of oxide redox pair materials for thermochemical cycling with the inherent effective heat transfer characteristics of foam structures and promotes them one step further to the development of an integrated receiver/reactor/heat exchanger configuration with enhanced heat storage characteristics.
In the first stages of the project, parametric Thermo-Gravimetric Analysis (TGA) on powders of all the redox pair oxides above and their combinations, exploring operational temperature range, longevity, limitations and peculiarities of redox operation run until 100 consecutive cycles, identified among the systems tested the redox pair of cobalt oxides Co3O4/CoO as the most suitable one since it can operate in a completely reversible, long-term, cyclic redox mode within the temperature range 800-1000oC.
Based on these findings, at first a variety of laboratory-scale, redox-inert ceramic foams and honeycombs were coated with Co3O4, spanning a wide range of loading percentages and tested in TGA experiments. When coated on oxide supports, full reduction/oxidation was observed even at very high Co3O4 loading percentages reaching 200 wt%. Long-term operation of such systems was tested successfully up to 100 consecutive cycles, at the end of which neither coating detachment nor any integrity loss of the coating-support assembly nor detrimental interaction of the redox coating with any of the oxide supports tested, was observed, whereas the extent of reduction and oxidation remained practically constant.
Next, it was shown that the manufacture of small-scale foams entirely of Co3O4 was possible. Such foams exhibited satisfactory structural integrity to be tested in cyclic reduction/oxidation conditions in a TGA apparatus. Foams made entirely of Co3O4 when tested up to 30 redox cycles maintained simultaneously their structural integrity and stoichiometric redox performance in contrast to (denser) pellets made also entirely from Co3O4 that cracked only after few cycles. This observation lead to dilatometry experiments which revealed that during redox cycling, “chemically”-induced stresses are developed due to the expansion/contraction of the cobalt oxide lattice during oxygen release/uptake respectively. These stresses are superimposed to “thermal-only” ones due to temperature cycling and under certain circumstances can lead to structure deformation and fracture. Based on this fact as well as on comparative estimates with respect to the volumetric energy storage density among the various structures (honeycombs, foams, pellets), it can be concluded that “open” porous structures made entirely of Co3O4 like the particular foams proposed and tested in the project can be an optimal choice since their large void space can reversibly accommodate and “buffer” the large volume expansion much better.
Subsequently, cascades of larger (1-inch diameter) porous ceramic honeycombs and foams have been first tested with respect to their sensible-only storage capability in a specially built furnace test rig, then coated at various loadings with Co3O4 and comparatively tested with respect to sensible/thermochemical storage capability. Parametric studies involved quantitative comparison of such hybrid sensible/thermochemical storage systems vs. respective sensible-only ones via the measurements of air stream exit temperatures as a function of the oxygen uptake/release, air flow rate and kind of porous support and redox material employed. Finally, a solar-irradiated receiver–heat storage module cascade was built to be used with specimens of 90 mm-diameter under direct solar irradiation conditions.
The construction modularity of the already existing current state-of-the-art sensible storage systems provides further for the implementation of spatial variation of redox oxide materials chemistry and solid materials porosity along the reactor/heat exchanger, to enhance the utilization of the heat transfer fluid and the storage of its enthalpy. Based on this characteristic, the concept of Cascaded Thermochemical Storage was conceived and the respective IPR were filed. The idea is based on employing cascades of various porous structures, incorporating different redox oxide materials and distributed in a certain rational pattern in space tailored to their thermochemical characteristics and the local temperature of the heat transfer medium. This concept can maximize the amount of redox material that can be efficiently exploited for thermochemical reactions, enhancing thereby the storage module’s volumetric heat storage capacity, transport, thermal and heat recovery properties and extending a solar plant’s off-sun operation beyond the current levels.
The most important impact of the project is the potential improvement of performance, flexibility and economics of power production by Concentrated Solar Power (CSP), especially Solar Thermal Tower power plants employing air as the heat transfer medium. The first such experimental 1.5 MWe plant, Solar Tower Jülich (STJ) that commenced in 2005, has already reached the status of commercial exploitation being currently in operation. The role of efficient thermal storage systems in materializing the prospects for future development and further commercialization of such plants cannot be underestimated. CSP has an inherent capacity to store heat energy for short periods of time for later conversion to electricity. CSP plants with large storage capacities may be able to produce base-load solar electricity day and night, making it possible for low-carbon CSP plants to compete with coal-fired power plants that emit high levels of CO2. Enhanced thermal storage would help to guarantee capacity and expand production, potentially making baseload solar-only power plants possible. The project results, aiming to extend the off-sun operation duration of such plants much beyond their current limits, are in full accordance with all these objectives. The utilization of redox materials as a means of energy storage can offer significant advantages compared to the ‘‘standard’’ storage approaches, provided that certain ‘‘bottlenecks’’ are overcome. The innovations proposed and their ‘‘fall-back’’ options aim at the elimination of these weaknesses so that the commercialization of this concept will become feasible at an industrial scale in the near-to-mid-term future. Such an outcome will have tremendous impact on the development and further commercialization of such plants. A drastic decrease on storage cost is expected, due to both the compactness and high efficiency of the proposed concept, paving the way for fully dispatchable power generation.
Addressing the major technical challenge for TCS applications which is the proper design and operation of TCS reactors that will operate simultaneously and efficiently as heat exchangers, the project has set forth and implemented for the first time the use of highly porous structures – ceramic foams made partially or entirely from redox oxide materials – as the medium within which the reaction and the heat exchange will take place. The proposed concept combines the demonstrated capabilities of oxide redox pair materials for thermochemical cycling with the inherent effective heat transfer characteristics of foam structures and promotes them one step further to the development of an integrated receiver/reactor/heat exchanger configuration with enhanced heat storage characteristics.
In the first stages of the project, parametric Thermo-Gravimetric Analysis (TGA) on powders of all the redox pair oxides above and their combinations, exploring operational temperature range, longevity, limitations and peculiarities of redox operation run until 100 consecutive cycles, identified among the systems tested the redox pair of cobalt oxides Co3O4/CoO as the most suitable one since it can operate in a completely reversible, long-term, cyclic redox mode within the temperature range 800-1000oC.
Based on these findings, at first a variety of laboratory-scale, redox-inert ceramic foams and honeycombs were coated with Co3O4, spanning a wide range of loading percentages and tested in TGA experiments. When coated on oxide supports, full reduction/oxidation was observed even at very high Co3O4 loading percentages reaching 200 wt%. Long-term operation of such systems was tested successfully up to 100 consecutive cycles, at the end of which neither coating detachment nor any integrity loss of the coating-support assembly nor detrimental interaction of the redox coating with any of the oxide supports tested, was observed, whereas the extent of reduction and oxidation remained practically constant.
Next, it was shown that the manufacture of small-scale foams entirely of Co3O4 was possible. Such foams exhibited satisfactory structural integrity to be tested in cyclic reduction/oxidation conditions in a TGA apparatus. Foams made entirely of Co3O4 when tested up to 30 redox cycles maintained simultaneously their structural integrity and stoichiometric redox performance in contrast to (denser) pellets made also entirely from Co3O4 that cracked only after few cycles. This observation lead to dilatometry experiments which revealed that during redox cycling, “chemically”-induced stresses are developed due to the expansion/contraction of the cobalt oxide lattice during oxygen release/uptake respectively. These stresses are superimposed to “thermal-only” ones due to temperature cycling and under certain circumstances can lead to structure deformation and fracture. Based on this fact as well as on comparative estimates with respect to the volumetric energy storage density among the various structures (honeycombs, foams, pellets), it can be concluded that “open” porous structures made entirely of Co3O4 like the particular foams proposed and tested in the project can be an optimal choice since their large void space can reversibly accommodate and “buffer” the large volume expansion much better.
Subsequently, cascades of larger (1-inch diameter) porous ceramic honeycombs and foams have been first tested with respect to their sensible-only storage capability in a specially built furnace test rig, then coated at various loadings with Co3O4 and comparatively tested with respect to sensible/thermochemical storage capability. Parametric studies involved quantitative comparison of such hybrid sensible/thermochemical storage systems vs. respective sensible-only ones via the measurements of air stream exit temperatures as a function of the oxygen uptake/release, air flow rate and kind of porous support and redox material employed. Finally, a solar-irradiated receiver–heat storage module cascade was built to be used with specimens of 90 mm-diameter under direct solar irradiation conditions.
The construction modularity of the already existing current state-of-the-art sensible storage systems provides further for the implementation of spatial variation of redox oxide materials chemistry and solid materials porosity along the reactor/heat exchanger, to enhance the utilization of the heat transfer fluid and the storage of its enthalpy. Based on this characteristic, the concept of Cascaded Thermochemical Storage was conceived and the respective IPR were filed. The idea is based on employing cascades of various porous structures, incorporating different redox oxide materials and distributed in a certain rational pattern in space tailored to their thermochemical characteristics and the local temperature of the heat transfer medium. This concept can maximize the amount of redox material that can be efficiently exploited for thermochemical reactions, enhancing thereby the storage module’s volumetric heat storage capacity, transport, thermal and heat recovery properties and extending a solar plant’s off-sun operation beyond the current levels.
The most important impact of the project is the potential improvement of performance, flexibility and economics of power production by Concentrated Solar Power (CSP), especially Solar Thermal Tower power plants employing air as the heat transfer medium. The first such experimental 1.5 MWe plant, Solar Tower Jülich (STJ) that commenced in 2005, has already reached the status of commercial exploitation being currently in operation. The role of efficient thermal storage systems in materializing the prospects for future development and further commercialization of such plants cannot be underestimated. CSP has an inherent capacity to store heat energy for short periods of time for later conversion to electricity. CSP plants with large storage capacities may be able to produce base-load solar electricity day and night, making it possible for low-carbon CSP plants to compete with coal-fired power plants that emit high levels of CO2. Enhanced thermal storage would help to guarantee capacity and expand production, potentially making baseload solar-only power plants possible. The project results, aiming to extend the off-sun operation duration of such plants much beyond their current limits, are in full accordance with all these objectives. The utilization of redox materials as a means of energy storage can offer significant advantages compared to the ‘‘standard’’ storage approaches, provided that certain ‘‘bottlenecks’’ are overcome. The innovations proposed and their ‘‘fall-back’’ options aim at the elimination of these weaknesses so that the commercialization of this concept will become feasible at an industrial scale in the near-to-mid-term future. Such an outcome will have tremendous impact on the development and further commercialization of such plants. A drastic decrease on storage cost is expected, due to both the compactness and high efficiency of the proposed concept, paving the way for fully dispatchable power generation.