Periodic Reporting for period 2 - ABraytCSPfuture (Air-Brayton cycle concentrated solar power future plants via redox oxides-based structured thermochemical heat exchangers/thermal boosters)
Reporting period: 2024-03-01 to 2025-06-30
Concentrated Solar Power (CSP) plants track the sun and concentrate its radiation by special mirrors converting solar energy to heat and via a power cycle to electricity. Eventual production of electricity via steam cycle limits the thermal-to-electric conversion efficiencies of CSP plants between 30 - 40%, thereby hampering CSP from becoming competitive to low-cost PVs. By increasing the maximum cycle temperature to 850–1000oC efficiencies of 53% would be achievable in an air-Brayton/Rankine Combined Cycle giving CSP a unique competitive edge.
For 24/7 power supply, a thermal energy storage (TES) system to provide heat when sun is unavailable but demand for electricity remains, is a “must”. The temperature level of the stored heat is limited by the thermal resilience of the HTF, being currently ca. 560/450 oC for state-of-the-art molten salt or thermal oil heat storage units, respectively.
The objectives of ABraytCSPfuture are to demonstrate an innovative, carbon-neutral way for implementing into air-operated CSP plants the much more efficient air-Brayton gas turbine cycle, increasing in parallel the plants’ TES capability by rendering their current sensible-only regenerative storage systems to hybrid sensible-Thermochemical storage (TCS) ones. Both will be achieved by exploiting reversible reduction/oxidation (redox) reaction schemes operating between the oxidized and reduced form of a metal oxide. In the thermal reduction step, the higher-valence oxide state under supply of external heat releases oxygen and transforms to the lower-valence form. Exothermic oxidation of the reduced to the oxidized form via air establishes a cyclic, solar energy TCS process. Materials of choice are such of a perovskite structure, comprising low cost, abundant metals. The ambition is to demonstrate, for the first time ever and at a proof-of-concept level, this radically new idea by the development and operation of a first-of-its-kind, compact, dual-bed thermochemical reactor/heat exchanger, comprised of non-moving, flow-through porous ceramic structures based on such perovskite redox oxides, raising in parallel the temperature to levels required for gas turbine cycles by performing the exothermic oxidation with pressurized air shifting its equilibrium to higher temperatures.
For 24/7 power supply, a thermal energy storage (TES) system to provide heat when sun is unavailable but demand for electricity remains, is a “must”. The temperature level of the stored heat is limited by the thermal resilience of the HTF, being currently ca. 560/450 oC for state-of-the-art molten salt or thermal oil heat storage units, respectively.
The objectives of ABraytCSPfuture are to demonstrate an innovative, carbon-neutral way for implementing into air-operated CSP plants the much more efficient air-Brayton gas turbine cycle, increasing in parallel the plants’ TES capability by rendering their current sensible-only regenerative storage systems to hybrid sensible-Thermochemical storage (TCS) ones. Both will be achieved by exploiting reversible reduction/oxidation (redox) reaction schemes operating between the oxidized and reduced form of a metal oxide. In the thermal reduction step, the higher-valence oxide state under supply of external heat releases oxygen and transforms to the lower-valence form. Exothermic oxidation of the reduced to the oxidized form via air establishes a cyclic, solar energy TCS process. Materials of choice are such of a perovskite structure, comprising low cost, abundant metals. The ambition is to demonstrate, for the first time ever and at a proof-of-concept level, this radically new idea by the development and operation of a first-of-its-kind, compact, dual-bed thermochemical reactor/heat exchanger, comprised of non-moving, flow-through porous ceramic structures based on such perovskite redox oxides, raising in parallel the temperature to levels required for gas turbine cycles by performing the exothermic oxidation with pressurized air shifting its equilibrium to higher temperatures.
• Computational materials screening tools and methodologies were further exploited to understand how composition (i.e. doping the CaMnO3 (CMO) structure with different cations such as Sr in the A-site and Fe in the B-site) affects thermal and mechanical properties of interest and tune them.
• Qualified, from the 1st reporting period and additional studies in the 2nd period, compositions (notably Sr-doped CMO) were further studied and synthesis conditions were optimized to ensure high purity of perovskite phase while reducing the energy requirements for the synthesis.
• Important properties of CMO-based materials, such as specific heat capacity and thermal conductivity, were measured and/or estimated to establish a solid basis for the necessary energetic calculations and modeling activities. Moreover, dilatometry measurements were carried out to correlate the effect of doping on the reversible expansion-contraction of the material during thermal/thermochemical operation in the 300-1100oC range.
• It was demonstrated that Sr-doped CMO lab-scale structures of both foam- and honeycomb-like formulation are able to substantially surpass the target of 300 kWh/m3 target of energy density and withstand the thermal/chemical stresses induced by the multi-cyclic operation (> 200 cycles) in the 300-1100oC range. First scaled-up production efforts provided confidence that, despite the challenges to be resolved, honeycomb-like structures can be produced at sizes suitable for the targeted prototype unit.
• First versions of reactor modeling/operation simulation advanced computational tools, appropriately refined with experimentally measured/produced geometric characteristics, thermophysical properties and reaction kinetics, were developed to facilitate the design of the prototype unit and predict its operation.
• A first version of Comparative Life Cycle Assessment (LCA) study was completed and determined the main characteristics and challenges/ addressing both material- and manufacturing-oriented aspects. With this as basis and considering additional experimental data gathered or currently underway, results will be further refined and further parametric analysis will be carried out to realistically define the boundaries and requirements to achieve sustainability of the envisaged ABCSPF system.
• Qualified, from the 1st reporting period and additional studies in the 2nd period, compositions (notably Sr-doped CMO) were further studied and synthesis conditions were optimized to ensure high purity of perovskite phase while reducing the energy requirements for the synthesis.
• Important properties of CMO-based materials, such as specific heat capacity and thermal conductivity, were measured and/or estimated to establish a solid basis for the necessary energetic calculations and modeling activities. Moreover, dilatometry measurements were carried out to correlate the effect of doping on the reversible expansion-contraction of the material during thermal/thermochemical operation in the 300-1100oC range.
• It was demonstrated that Sr-doped CMO lab-scale structures of both foam- and honeycomb-like formulation are able to substantially surpass the target of 300 kWh/m3 target of energy density and withstand the thermal/chemical stresses induced by the multi-cyclic operation (> 200 cycles) in the 300-1100oC range. First scaled-up production efforts provided confidence that, despite the challenges to be resolved, honeycomb-like structures can be produced at sizes suitable for the targeted prototype unit.
• First versions of reactor modeling/operation simulation advanced computational tools, appropriately refined with experimentally measured/produced geometric characteristics, thermophysical properties and reaction kinetics, were developed to facilitate the design of the prototype unit and predict its operation.
• A first version of Comparative Life Cycle Assessment (LCA) study was completed and determined the main characteristics and challenges/ addressing both material- and manufacturing-oriented aspects. With this as basis and considering additional experimental data gathered or currently underway, results will be further refined and further parametric analysis will be carried out to realistically define the boundaries and requirements to achieve sustainability of the envisaged ABCSPF system.
• Sr-doped lab-scale structured bodies were tested in dedicated test rigs and were found to have energy density of up to 840 kWh/m3 based on experimental measurements obtained during their evaluation and associated straightforward energetic calculations. These values are more than double of those of current state-of-the-art sensible-storage materials used in CSP, namely molten salts that are of the order of 200 kWh/m3 and well above the project’s target of 300 kWh/m3.
• In the course of the aforementioned experimental campaigns, the structures studied (foams, honeycombs) were confirmed to be rigid and thermally stable in the course of > 200 cycles, as demonstrated by relevant macroscopic and microscopic observations and measurements of aged vs pristine samples.
• Scaled-up structures, primarily honeycombs but also foams, were successfully prepared from Sr-doped CMO perovskites and lab-scale specimens sectioned from them demonstrated that their performance was identical to the one of corresponding small-scale manufactured samples. To the best of the partners’ knowledge this is the first time that a production of such scaled-up structured redox perovskite ceramics is reported in the open literature.
• A simple redox kinetic model, able to fairly predict the experimental data, was implemented and directly used by a global first version of the envisaged storage reactor/heat exchanger model. This integrated modeling activity, enriched by realistic experimental measurements of key properties, provided a meaningful approach for the design of a future scaled-up system and facilitated the definition of boundary conditions, limitations and requirements of the ABCSPF technology.
• In the course of the aforementioned experimental campaigns, the structures studied (foams, honeycombs) were confirmed to be rigid and thermally stable in the course of > 200 cycles, as demonstrated by relevant macroscopic and microscopic observations and measurements of aged vs pristine samples.
• Scaled-up structures, primarily honeycombs but also foams, were successfully prepared from Sr-doped CMO perovskites and lab-scale specimens sectioned from them demonstrated that their performance was identical to the one of corresponding small-scale manufactured samples. To the best of the partners’ knowledge this is the first time that a production of such scaled-up structured redox perovskite ceramics is reported in the open literature.
• A simple redox kinetic model, able to fairly predict the experimental data, was implemented and directly used by a global first version of the envisaged storage reactor/heat exchanger model. This integrated modeling activity, enriched by realistic experimental measurements of key properties, provided a meaningful approach for the design of a future scaled-up system and facilitated the definition of boundary conditions, limitations and requirements of the ABCSPF technology.