• Energy Research

Molten carbonate salt nanofluids have become an excellent strategy to enhance the power generation efficiency of the next-generation concentrated solar power (CSP) technology due to their excellent thermophysical properties at high temperatures. The corrosion behaviour in contact with salt nanofluids is one of the main issues to address for implementing cost-effective steels as construction materials for CSP. In the present work, the effects of molten salt nanofluids with Al2O3 nanoparticles (<50 nm and 1.0 wt%) on the corrosion behaviour of AISI 301LN stainless steel have been investigated. Corrosion tests were carried out in a static molten salt mixture of Li2CO3–Na2CO3–K2CO3 at 600 °C for 1000 h. Complementary electron microscopy, microanalysis and optical characterization techniques and micromechanical tests were carried out to study the oxide scales formed after corrosion tests in salt nanofluids and base salt of reference. Results evidenced that doping nanoparticles in molten salts significantly decreased the corrosion rate by ~50%, compared with a base salt. Incorporating nanoparticles into the oxide scale, forming a nanostructure of reticulated Al2O3 nanoparticles generated a more homogeneous and dense oxide scale, increasing its hardness about 30% and enhancing its effectiveness as a protective barrier. Peer Reviewed

Next generation concentrated solar power (CSP) plants promise a higher operating temperature and better efficiency. However, new issues related to the corrosion against protection of the construction alloys need to be solved. In this work, two different duplex stainless steels grades, namely 2205 (DS2205) and 2507 (DS2507), were evaluated for their compatibility with the eutectic molten salt mixture of Li2CO3-K2CO3-Na2CO3 at 500 °C in air for thermal energy storage applications. Corrosion tests combined with complementary microscopy, microanalysis and mechanical techniques were employed to study the oxide scales formed on the surface of the duplex steels. The corrosion tests evidenced that the attack morphology in both duplex steels was a uniform oxidative process without localized corrosion. DS2507 presented a better corrosion resistance than DS2205, due to the formation of thinner, compact and continuous oxide layers with higher compositional content in Cr, Ni and Mo than DS2205. The oxide scales of DS2507 showed more remarkable mechanical integrity and adhesion to the metallic substrate.

Next-generation concentrated solar power (CSP) plants are required to operate at temperatures as high as possible to reach a better energy efficiency. This means significant challenges for the construction materials in terms of corrosion resistance, among others. In the present work, the corrosion behavior in a molten eutectic ternary Li2CO3-Na2CO3-K2CO3 mixture at 600 °C was studied for three stainless steels: an austenitic grade AISI 301LN (SS301) and two duplex grades, namely 2205 (DS2205) and 2507 (DS2507). Corrosion tests combined with complementary microscopy, microanalysis and mechanical characterization techniques were employed to determine the corrosion kinetics of the steels and the oxide scales formed on the surface. The results showed that all three materials exhibited a corrosion kinetics close to a parabolic law, and their corrosion rates increased in the following order: DS2507 < SS301 < DS2205. The analyses of the oxide scales evidenced an arranged multilayer system with LiFeO2, LiCrO2, FeCr2O4 and NiO as the main compounds. While the Ni-rich inner layer of the scales presented a good adhesion to the metallic substrate, the outer layer formed by LiFeO2 exhibited a higher concentration of porosity and voids. Both the Cr and Ni contents at the inner layer and the defects at the outer layer were crucial for the corrosion resistance for each steel. Among the studied materials, super duplex stainless steel 2507 is found to be the most promising alternative for thermal energy storage of those structural components for CSP plants.

Abstract Fuel electrode supported solid oxide cells (SOCs) have been developed on an industrial scale using the aqueous tape-casting technique. The NiO–yttria-stabilized zirconia Y2O3–ZrO2 (YSZ) fuel electrode and YSZ electrolyte have been manufactured by multilayer co-laminated tape casting. Details of the tape-casting slurry formulations are described and discussed. Two types of cells were fabricated with different microstructures of the NiO–YSZ support discussed. Good electrochemical performance and stability in SOFC mode at 750 °C and 0.7 V for both button cells reaching around >0.75 W cm−2 and with no measurable degradation after >700 h were observed. The selected cell was scaled up to large-area cells (36 cm2 of the active area) and electrochemically tested at 750 °C in a single repetition unit (SRU) in SOFC (Solid Oxide Fuel Cell), SOEC (Solid Oxide Electrolysis Cell) and co-SOEC (Solid Oxide co-Electrolysis Cell) mode, and in a short-stack of two SRUs in SOFC mode. A current up to 17 A was obtained at 1.4 V (0.7 V cell−1) with the short-stack configuration in SOFC mode, corresponding to ∼0.5 A cm−2 and 24 W. The performances of the aqueous-based SOC cells can be considered highly remarkable, thus supporting the success in scaling the fabrication of SOC stacks using more environmentally friendly processes than conventional ones.

Next-generation concentrated solar power (CSP) technology needs to work at high temperatures to increase its power generation efficiency. It represents overcoming some issues for the structural materials, such as corrosion mitigation. In the present work, the laser-surface texturing (LST) treatment is investigated as a surface modification method to mitigate corrosion of a duplex stainless steel for CSP structural material. Corrosion tests of duplex steel were carried out immersed in a static molten salt mixture of Li2CO3-Na2CO3-K2CO3 at 600 °C for 1000 h. Complementary microscopy, microanalysis and optical characterization techniques were used to analyse both the steel surface after LST and the oxide scales formed after corrosion tests. Results evidenced that the laser treatment favoured the formation of a denser oxide layer, improving its effectiveness as a protective barrier against further oxidation. Compared with an untreated surface, the laser-treated one significantly decreased the corrosion rate by 48 %. This research was funded by Direcció General d'Indústria and ACCIÓ (Generalitat de Catalunya) to IMEM-CIEFMA in the INNOTEC project with grant number ACE034/21/000031, and Agency for Administration of University and Research (Agència de Gestió d'Ajuts Universitaris i de Recerca, AGAUR) (2021 SGR 01053). Mohammad Rezayat also acknowledges the AGAUR Fellowship (FI-SDUR-2020) of the Generalitat de Catalunya for its financial support. Peer Reviewed

Abstract Fuel Cells (FCs) are electrochemical devices that directly convert the chemical energy of a reaction into electrical energy. The energy conversion processes theoretically remain unaltered as long as the fuel and oxidant feed the device. A single cell consists of two electrodes (anode and cathode), and an electrolyte, which can be a solid oxide that transports ions, thus becoming a SOFC (solid oxide fuel cell). To achieve the oxide-ion conductivity necessary to ensure high enough power density, these fuel cells require high and/or intermediate temperatures (over 500 °C).The fuel conversion efficiency of conventional SOFCs is usually about 50%. Thus, much of the chemical energy converts into waste heat energy, whereas thermoelectric materials can generate electricity from the waste heat. Some studies have been published in which the cathode of the fuel cell has been replaced by a thermoelectric material, and different simulation studies have been performed in which the waste heat is harnessed by thermoelectrics in a heat exchanger. The aim of this work is to provide an overview of thermoelectric materials that could help to select the best one to harness the heat evolved by a SOFC device to increase its efficiency. Data has been collected for different thermoelectric materials, including their thermoelectric performance, operation temperature range, and cost. To choose a thermoelectric material, it is necessary to define a performance parameter that allows us to classify all the different materials by means of their performance. Among the best, operating temperature and cost will turn into constraints that must be met to find the best material that suits the characteristics of a SOFC operation environment, and that really can be used in combination with the fuel cell.