Abstract
To improve the thermochemical energy storage (TCS) behavior of Mn2O3, several Mn–Mo oxides with varying amounts of MoO3 (0–30 wt%) were prepared by a precipitation method. The physico-chemical properties of the solids were studied by N2 adsorption–desorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), and H2-temperature-programmed reduction (TPR), while their TCS behavior was determined by thermogravimetric analysis coupled with differential scanning calorimetry (TGA-DSC). Apart from Mn2O3 and MoO3 phases, XRD revealed a mixed MnMoO4 phase for MoO3 loadings equal or higher than 1.5 wt%. All samples showed a well-formed coral-like surface morphology, particularly those solids with low MoO3 contents. This coral morphology was progressively decorated with compact and Mo-enriched MnMoO4 particles as the MoO3 content increased. TPR revealed that the redox behavior of Mn2O3 was significantly altered upon addition of Mo. The TCS behavior of Mn2O3 (mostly oxidation kinetics and redox cyclability) was enhanced by addition of low amounts of Mo (0.6 and 1.5% MoO3) without significantly increasing the reduction temperature of the solids. The coral morphology (which facilitated oxygen diffusion) and a smoother transition from the reduced to oxidized phase were suggested to be responsible for this improved TCS behavior. The samples containing 0.6 and 1.5 wt% of MoO3 showed outstanding cyclability after 45 consecutive reduction–oxidation cycles at high temperatures (600–1000 °C). These materials could potentially reach absorption efficiencies higher than 90% at concentration capacity values typical of concentrated solar power plants.
Highlights
Global energy consumption is increasing by the ever-growing demand of energy from developing countries, and renewable energy sources are becoming more important in covering this demand [1]
While high discharge temperatures allow increasing the efficiency of steam turbines, high energy densities are important for reducing the size of the storage tank facilities and, decreasing the capital expenses of the plant [3,5,6]
We considered the following process to describe the thermochemical behavior of Mn–Mo mixed oxides: MoxMn2−2·xO3
Summary
Global energy consumption is increasing by the ever-growing demand of energy from developing countries, and renewable energy sources are becoming more important in covering this demand [1]. In the case of CSP, the development of efficient thermal energy storage (TES) systems is important. Three different TES approaches based on sensible, latent, and thermochemical heat are currently available. Thermochemical energy storage (TCS, i.e., the utilization of a reversible chemical process to store thermal energy) holds great promise for future CSP plants owing to its higher energy density and discharge temperatures compared to sensible- and latent heat-based approaches. While high discharge temperatures allow increasing the efficiency of steam turbines, high energy densities are important for reducing the size of the storage tank facilities and, decreasing the capital expenses of the plant [3,5,6]. TCS allows solar thermal energy to be stored (and subsequently released) in form of chemical energy by promoting a high-temperature reversible chemical process. As chemical bonds are broken and formed during the process, large amounts of energy are potentially absorbed/released within each cycle [7,8]
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