RESUMEN
The utilization of the Solar Salt (60 wt%NaNO3/40 wt%KNO3) mixture as a Thermal Energy Storage (TES) medium is gaining importance due to its scalability and cost-effectiveness. However, the corrosion of metallic components presents a significant challenge. This study explores the intricate interplay between salt chemistry and its corrosivity, particularly at elevated temperatures exceeding the state-of-the-art bulk temperature 565 °C. The study manipulates salt decomposition by adjusting the oxygen partial pressure in the purge gas over Solar Salt and investigates the evolution of salt chemistry with and without the presence of steel. It analyzes the corrosion behavior of two types of stainless steel, AISI 316L and AISI 310, under different gas purging atmospheres. Furthermore, it employs a gold particle tracing technique to identify and monitor the formation and growth of the corrosion layer on the steel surface. The results reveal that nitrogen gas purging significantly enhances salt decomposition and its corrosivity over time. The presence of steel also influences salt decomposition depending on the purged gas atmosphere. In a nitrogen atmosphere, the presence of steel can increase the nitrite levels, while an air atmosphere results in an elevated concentration of oxide ions. In air, the AISI 310 alloy shows slightly better performance than AISI 316L. Both alloys experience substantial mass loss in the nitrogen-purged atmosphere. Interestingly, the presence of gold particles within the middle of the corrosion layer in the air purged atmosphere visually illustrates a counter diffusion involving various cations and anions across the corrosion layer.
RESUMEN
Among the variety of energy storage techniques thermal energy storage (TES), based on molten salts, is already in use for the storage of heat in a gigawatt hour scale. At the time of writing virtually all TES in CSP utilize Solar Salt (60 wt-% NaNO3 and 40 wt-% KNO3) due to its competitively low price, low vapor pressure and non-toxicity. On the downside, the operating temperature is limited to 560 °C based on its thermal stability. However, increasing the operating temperature while maintaining thermal stability of the salt using techniques that are realizable in industrial scale remains one of the main challenges. Up to now this could only be achieved in a small scale by flushing with synthetic purge gas or sealing and pressurizing the system, maintaining the necessary gas atmosphere and shifting the chemical equilibrium to the nitrate side. Both methods are hardly realizable in an industrial scale. In this work we show a new strategy to stabilize Solar Salt at 620 °C by combining the gas-purged configuration and sealed system with maximum pressure of few tens of millibars in a 100 kg scale. The formed gas phase was within the expected range in terms of oxygen and nitrous gases. Additionally, the concentration of the nitrate and nitrite ions aligned well with salt systems with gas-purged atmosphere at 620 °C. We demonstrate the first experiments on long-term thermal stabilization (4000 h) of Solar Salt at 620 °C in a 100 kg technical-scale. These findings represent an important step in the development of modern storage systems.
RESUMEN
The scope of our study was to examine the potential of regeneration mechanisms of an aged molten Solar Salt (nitrite, oxide impurity) by utilization of reactive gas species (nitrous gases, oxygen). Initially, aging of Solar Salt (60 wt% NaNO3, 40 wt% KNO3) was mimicked by supplementing the decomposition products, sodium nitrite and sodium peroxide, to the nitrate salt mixture. The impact of different reactive purge gas compositions on the regeneration of Solar Salt was elaborated. Purging the molten salt with a synthetic air (p(O2) = 0.2 atm) gas stream containing NO (200 ppm), the oxide ion concentration was effectively reduced. Increasing the oxygen partial pressure (p(O2) = 0.8 atm, 200 ppm NO) resulted in even lower oxide ion equilibrium concentrations. To our knowledge, this investigation is the first to present evidence of the regeneration of an oxide rich molten Solar Salt, and reveals the huge impact of reactive gases on Solar Salt reaction chemistry.
RESUMEN
An in situ/operando flow cell for transmission mode X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and combined XAS/XRD measurements in a single experiment under the extreme conditions of two-step solar thermochemical looping for the dissociation of water and/or carbon dioxide was developed. The apparatus exposes materials to relevant conditions of both the auto-reduction and the oxidation sub-steps of the thermochemical cycle at ambient temperature up to 1773 K and enables determination of the composition of the effluent gases by online quadrupole mass spectrometry. The cell is based on a tube-in-tube design and is heated by means of a focusing infrared furnace. It was tested successfully for carbon dioxide splitting. In combined XAS/XRD experiments with an unfocused beam, XAS measurements were performed at the Ce K edge (40.4 keV) and XRD measurements at 64.8 keV and 55.9 keV. Furthermore, XRD measurements with a focused beam at 41.5 keV were carried out. Equimolar ceria-hafnia was auto-reduced in a flow of argon and chemically reduced in a flow of hydrogen/helium. Under reducing conditions, all cerium(iv) was converted to cerium(iii) and a cation-ordered pyrochlore-type structure was formed, which was not stable upon oxidation in a flow of carbon dioxide.
RESUMEN
X-ray absorption spectroscopy was used to characterise ceria-based materials under realistic conditions present in a reactor for solar thermochemical two-step water and carbon dioxide splitting. A setup suitable for in situ measurements in transmission mode at the cerium K edge from room temperature up to 1773 K is presented. Time-resolved X-ray absorption near-edge structure (XANES) data, collected for a 10 mol% hafnium-doped ceria sample (Ce0.9Hf0.1O2-δ) during reduction at 1773 K in a flow of inert gas and during re-oxidation by CO2 at 1073 K, enables the quantitative determination of the non-stoichiometry δ of the fluorite-type structure. XANES analysis suggests the formation of the hexagonal Ce2O3 phase upon reduction in 2% hydrogen/helium at 1773 K. We discuss the experimental limitations and possibilities of high-temperature in situ XAS at edges of lower energy as well as the importance of the technique for understanding and improving the properties of ceria-based oxygen storage materials for thermochemical solar energy conversion.
RESUMEN
Efficient heat transfer of concentrated solar energy and rapid chemical kinetics are desired characteristics of solar thermochemical redox cycles for splitting CO2. We have fabricated reticulated porous ceramic (foam-type) structures made of ceria with dual-scale porosity in the millimeter and micrometer ranges. The larger void size range, with dmean = 2.5 mm and porosity = 0.76-0.82, enables volumetric absorption of concentrated solar radiation for efficient heat transfer to the reaction site during endothermic reduction, while the smaller void size range within the struts, with dmean = 10 µm and strut porosity = 0-0.44, increases the specific surface area for enhanced reaction kinetics during exothermic oxidation with CO2. Characterization is performed via mercury intrusion porosimetry, scanning electron microscopy, and thermogravimetric analysis (TGA). Samples are thermally reduced at 1773 K and subsequently oxidized with CO2 at temperatures in the range 873-1273 K. On average, CO production rates are ten times higher for samples with 0.44 strut porosity than for samples with non-porous struts. The oxidation rate scales with specific surface area and the apparent activation energy ranges from 90 to 135.7 kJ mol(-1). Twenty consecutive redox cycles exhibited stable CO production yield per cycle. Testing of the dual-scale RPC in a solar cavity-receiver exposed to high-flux thermal radiation (3.8 kW radiative power at 3015 suns) corroborated the superior performance observed in the TGA, yielding a shorter cycle time and a mean solar-to-fuel energy conversion efficiency of 1.72%.