High-temperature X-ray diffraction with concurrent gas chromatography (GC) was used to study cobalt disulfide cathode pellets disassembled from thermal batteries. When CoS2 cathode materials were analyzed in an air environment, oxidation of the K(Br, Cl) salt phase in the cathode led to the formation of K2SO4 that subsequently reacted with the pyrite-type CoS2 phase leading to cathode decomposition between ∼260 and 450 °C. Independent thermal analysis experiments, i.e. simultaneous thermogravimetric analysis/differential scanning calorimetry/mass spectrometry (MS), augmented the diffraction results and support the overall picture of CoS2 decomposition. Both gas analysis measurements (i.e. GC and MS) from the independent experiments confirmed the formation of SO2 off-gas species during breakdown of the CoS2. In contrast, characterization of the same cathode material under inert conditions showed the presence of CoS2 throughout the entire temperature range of analysis.
The use of perovskite oxides as a medium for thermochemical energy storage (TCES) in concentrating solar power systems is reported. The known reduction/oxidation (redox) active perovskites LaxSr1-xCoyMn1-yO3 (LSCM) and LaxSr1-xCoyFe1-yO3 (LSCF) were chosen as a starting point for such research. Materials of the LSCM and LSCF family were previously synthesized, their structure characterized, and thermodynamics reported for TCES operation. Building on this foundation, the reduction onset temperatures are examined for LSCM and LSCF compositions. The reduction extents and onset temperatures are tied to the crystallographic phase and reaction enthalpies. The effect of doping with Ba and K is discussed, and the potential shortcomings of this subset of materials families for TCES are described. The potential for long-term stability of the most promising material is examined through thermogravimetric cycling, scanning electron microscopy, and dilatometry. The stability over 100 cycles (450-1050 °C) of an LSCM composition is demonstrated.
Developing efficient thermal storage for concentrating solar power plants is essential to reducing the cost of generated electricity, extending or shifting the hours of operation, and facilitating renewable penetration into the grid. Perovskite materials of the CaBxMn1-xO3-δ family, where B=Al or Ti, promise improvements in cost and energy storage density over other perovskites currently under investigation. Thermogravimetric analysis of the thermal reduction and reoxidation of these materials was used to extract equilibrium thermodynamic parameters. The results demonstrate that these novel thermochemical energy storage media display the highest reaction enthalpy capacity for perovskites reported to date, with a reaction enthalpy of 390kJ/kg, a 56% increase over previously reported compositions.
ASME 2016 10th International Conference on Energy Sustainability, ES 2016, collocated with the ASME 2016 Power Conference and the ASME 2016 14th International Conference on Fuel Cell Science, Engineering and Technology
In an effort to increase thermal energy storage densities and turbine inlet temperatures in concentrating solar power (CSP) systems, focus on energy storage media has shifted from molten salts to solid particles. These solid particles are stable at temperatures far greater than that of molten salts, allowing the use of efficient high-temperature turbines in the power cycle. Furthermore, many of the solid particles under development store heat via reversible chemical reactions (thermochemical energy storage, TCES) in addition to the heat they store as sensible energy. The heat-storing reaction is often the thermal reduction of a metal oxide. If coupled to an Air-Brayton system, wherein air is used as the turbine working fluid, the subsequent extraction of both reaction and sensible heat, as well as the transfer of heat to the working fluid, can be accomplished in a direct-contact, counter-flow reoxidation reactor. However, there are several design challenges unique to such a reactor, such as maintaining requisite residence times for reactions to occur, particle conveying and mitigation of entrainment, and the balance of kinetics and heat transfer rates to achieve reactor outlet temperatures in excess of 1200 °C. In this paper, insights to addressing these challenges are offered, and design and operational tradeoffs that arise in this highlycoupled system are introduced and discussed.
ASME 2016 10th International Conference on Energy Sustainability, ES 2016, collocated with the ASME 2016 Power Conference and the ASME 2016 14th International Conference on Fuel Cell Science, Engineering and Technology
Thermochemical energy storage (TCES) offers the potential for greatly increased storage density relative to sensible-only energy storage. Moreover, heat may be stored indefinitely in the form of chemical bonds via TCES, accessed upon demand, and converted to heat at temperatures significantly higher than current solar thermal electricity production technology and is therefore well-suited to more efficient high-temperature power cycles. The PROMOTES effort seeks to advance both materials and systems for TCES through the development and demonstration of an innovative storage approach for solarized Air-Brayton power cycles and that is based on newly-developed redox-active metal oxides that are mixed ionic-electronic conductors (MIEC). In this paper we summarize the system concept and review our work to date towards developing materials and individual components.
Establishing processing-structure-property relationships for monolayer materials is crucial for a range of applications spanning optics, catalysis, electronics and energy. Presently, for molybdenum disulfide, a promising catalyst for artificial photosynthesis, considerable debate surrounds the structure/property relationships of its various allotropes. Here we unambiguously solve the structure of molybdenum disulfide monolayers using high-resolution transmission electron microscopy supported by density functional theory and show lithium intercalation to direct a preferential transformation of the basal plane from 2H (trigonal prismatic) to 1T′ (clustered Mo). These changes alter the energetics of molybdenum disulfide interactions with hydrogen (ΔG H), and, with respect to catalysis, the 1T′ transformation renders the normally inert basal plane amenable towards hydrogen adsorption and hydrogen evolution. Indeed, we show basal plane activation of 1T′ molybdenum disulfide and a lowering of ΔG H from +1.6 eV for 2H to +0.18 eV for 1T′, comparable to 2H molybdenum disulfide edges on Au(111), one of the most active hydrogen evolution catalysts known.
Materials in the LaxSr1–xCoyMn1–yO3–δ (LSCM) and LaxSr1–xCoyFe1–yO3–δ (LSCF) families are candidates for high-temperature thermochemical energy storage due to their facility for cyclic endothermic reduction and exothermic oxidation. A set of 16 LSCM and 21 LSCF compositions were synthesized by a modified Pechini method and characterized by powder X-ray diffraction and thermogravimetric analysis. All materials were found to be various symmetries of the perovskite phase. LSCM was indexed as tetragonal, cubic, rhombohedral, or orthorhombic as a function of increased lanthanum content. For LSCF, compositions containing low lanthanum content were indexed as cubic while materials with high lanthanum content were indexed as rhombohedral. An initial screening of redox activity was completed by thermogravimetric analysis for each composition. The top three compositions with the greatest recoverable redox capacity for each family were further characterized in equilibrium thermogravimetric experiments over a range of temperatures and oxygen partial pressures. As a result, these equilibrium experiments allowed the extraction of thermodynamic parameters for LSCM and LSCF compositions operated in thermochemical energy storage conditions.
The thermal properties of a commercial copper-diamond composite were measured from below -50°C to above 200°C. The results of thermal expansion, heat capacity, and thermal diffusivity were reported. These data were used to calculate the thermal conductivity of the composite as a function of temperature in the thickness direction. These results are compared with estimated values based on a simple mixing rule and the temperature dependence of these physical properties is represented by curve fitting equations. These fitting equations can be used for thermal modeling of practical devices/systems at their operation temperatures. The results of the mixing rule showed a consistent correlation between the amount of copper and diamond in the composite, based on density, thermal expansion, and heat capacity measurements. However, there was a disparity between measured and estimated thermal diffusivity and thermal conductivity. These discrepancies can be caused by many intrinsic material issues such as lattice defects and impurities, but the dominant factor is attributed to the large uncertainty of the interfacial thermal conductance between diamond and copper.