Publications

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A two-step cycle was considered for solar thermochemical energy storage based on aluminum-doped calcium manganite reduction/oxidation reactions for direct integration into Air Brayton cycles. The two steps encompassed (1) the storage of concentrated solar direct irradiation via the thermal reduction of aluminum-doped calcium manganite and (2) the delivery of heat to an Air-Brayton cycle via re-oxidation of oxygen-deficient aluminum-doped calcium manganite. The re-oxidized aluminum-doped calcium manganite was fed back to the first step to complete the cycle. A 5 kWth solar thermochemical reactor operating under vacuum was fabricated and tested to examine the first cycle reduction step. Reactor operating conditions and high-flux solar simulator control were tuned for continuous reactor operation with particle temperatures >1073 K. Continuous operation was achieved using intermittent, dense granular flows. A maximum absorption efficiency of 64.7% was demonstrated, accounting for both sensible and chemical heat storage.

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Pyromark® 2500, manufactured by Tempil, is currently the industry standard for high solar absorptive receiver coatings for concentrating solar power towers. However, Pyromark has been reported to degrade if not applied properly or exposed to temperatures exceeding 700 °C over a period of time. However, it is not apparent if such degradation is due to a particular aspect or aspects of the deposition process, which may vary from plant to plant. Many variables factor in to the performance of Pyromark, e.g. deposition method, drying time, curing parameters (ramp rate, homogeneous heating, time at temperature.), and coating thickness. Identifying the factors with the most influence on coating performance and durability will help guide the application of Pyromark to receivers to minimize degradation over time. The relationships between coating quality and optical properties with deposition/curing parameters on Haynes 230 substrates were assessed using statistical analysis of variance (ANOVA) techniques for repeated measures. These ANOVA techniques were designed to detect differences in treatment effects on the response at each of the aging cycles. The analyses found that coating thickness, curing ramp rate, and dwell time had the most effect on coating quality.

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A coupled electrochemical/thermochemical cycle was investigated to produce hydrogen from renewable resources. Like a conventional thermochemical cycle, this cycle leverages chemical energy stored in a thermochemical working material that is reduced thermally by solar energy. However, in this concept, the stored chemical energy only needs to be partially capable of splitting steam to produce hydrogen. To push the reaction to completion, a proton-conducting membrane is employed to separate hydrogen as it is produced, thus shifting the thermodynamics toward further hydrogen production. This novel coupled-cycle concept provides several benefits. First, the required oxidation enthalpy of the reversible thermochemical material is reduced, enabling the process to occur at lower temperatures. Second, removing the requirement for spontaneous steam splitting widens the scope of materials compositions, allowing for less expensive/more abundant elements to be used. Lastly, thermodynamics calculations suggest that this concept can potentially reach higher efficiencies than photovoltaic-to-electrolysis hydrogen production methods. This Exploratory Express LDRD involved assessing the practical feasibility of the proposed coupled cycle. A test stand was designed and constructed and proton-conducting membranes were synthesized. An LDRD plus-up of $10k enabled the remediation of a membrane sealing issue and enabled testing with an improved membrane. However, the membrane proved too thick for efficient proton conduction, and there were insufficient funds to continue. While the full proof of concept was not achieved, the individual components of the experiment were validated and new capabilities that can be leveraged by a variety of programs were developed.

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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. However, this potential has yet to be realized as no current TCES system satisfies all requirements. This project involves the design, development, and demonstration of a robust and innovative storage cycle based on redox-active metal oxides that are Mixed Ionic-Electronic Conductors (MIECs). We will develop, characterize, and demonstrate a first of its kind 100kWth particle-based TCES system for direct integration with combined-cycle Air Brayton based on the endothermic reduction and exothermic reoxidation of MIECs. Air Brayton cycles require temperatures in the range of 1000-1230 °C for smaller axial flow turbines and are therefore inaccessible to all but the most robust storage solutions such as metal oxides. The choice of MIECs, with exceptional tunability and stability over the specified operating conditions allows us to optimally target this high impact cycle and to introduce the innovation of directly driving the turbine with the reacting/heat recovery fluid. The potential for high temperature thermal storage has direct bearing on next-gen CSP, and an appropriate investment for SETO.

A coupled electrochemical/thermochemical cycle was investigated to produce hydrogen from renewable resources. Like a conventional thermochemical cycle, this cycle leverages chemical energy stored in a thermochemical working material that is reduced thermally by solar energy. However, in this concept, the stored chemical energy only needs to be partially, but not fully, capable of splitting steam to produce hydrogen. To complete the process, a proton-conducting membrane is driven to separate hydrogen as it is produced, thus shifting the thermodynamics toward further hydrogen production. This novel coupled-cycle concept provides several benefits. First, the required oxidation enthalpy of the reversible thermochemical material is reduced, enabling the process to occur at lower temperatures. Second, removing the requirement for spontaneous steam-splitting widens the scope of materials compositions, allowing for less expensive/more abundant elements to be used. Lastly, thermodynamics calculations suggest that this concept can potentially reach higher efficiencies than photovoltaic-to-electrolysis hydrogen production methods. This Exploratory Express LDRD involved assessing the practical feasibility of the proposed coupled cycle. A test stand was designed and constructed and proton-conducting membranes were synthesized. While the full proof of concept was not achieved, the individual components of the experiment were validated and new capabilities that can be leveraged by a variety of programs were developed.

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A novel concept for coupling a thermochemical cycle with an electrochemical separation device for the generation of hydrogen from steam is reported and a thermodynamic analysis of the system is presented. In a conventional thermochemical cycle, an oxygen carrier material is thermally reduced, cooled, and then reoxidized in steam thereby generating hydrogen. However, this process often requires high temperatures (>1700 K) and/or low oxygen partial pressures (<0.001 atm) in order to meet thermodynamic requirements. Such extreme conditions can adversely affect the stability of the reactive oxides, reactor materials, and system efficiency. In our proposed technology, we seek to decrease the required reduction temperature by several hundred degrees Kelvin by relaxing the requirement for spontaneous oxidation reaction at atmospheric pressure. This is accomplished by incorporating a proton-conducting membrane (PCM) to separate hydrogen produced at equilibrium concentrations from reactant steam. We also suggest the use of mixed ionic-electronic conducting (MIEC) oxygen carrier materials that reduce through a continuum of oxidation states at lower temperatures (∼1200 °C). This concept allows the generation of a high-quality hydrogen stream while avoiding the challenging high temperatures/low partial pressures required in conventional water-splitting reaction schemes.

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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.

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Over the course of this project, solar selective coatings intended for next-generation power tower applications were researched and developed. The aim of these coatings was to combine high absorptance with relatively low thermal emittance in materials and designs that could perform without significant degradation at ≥700 °C and 600 suns irradiance in air. This would surpass the reported performance of the industry’s benchmark material, Pyromark® 2500. SunShot goals require that tower receiver coatings be stable in air, have high thermal conductivity, and be nonvolatile. We considered deposition methods that can be scaled-up to practical sizes and we made careful analyses of the costs associated with our coatings and the requirements that would make them effective. We have conducted durability testing of promising materials deposited on appropriate Ni-superalloy substrates in conventional thermal furnaces, solar simulators, and on-sun testing to understand the absorption efficiency of the coatings, degradation mechanisms and stability under realistic simulated environments.

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.

The contribution of each component of a power generation plant to the levelized cost of energy (LCOE) can be estimated and used to increase the power output while reducing system operation and maintenance costs. The LCOE is used in order to quantify solar receiver coating influence on the LCOE of solar power towers. Two new parameters are introduced: the absolute levelized cost of coating (LCOC) and the LCOC efficiency. Depending on the material properties, aging, costs, and temperature, the absolute LCOC enables quantifying the cost-effectiveness of absorber coatings, as well as finding optimal operating conditions. The absolute LCOC is investigated for different hypothetic coatings and is demonstrated on Pyromark 2500 paint. Results show that absorber coatings yield lower LCOE values in most cases, even at significant costs. Optimal reapplication intervals range from one to five years. At receiver temperatures greater than 700 °C, non-selective coatings are not always worthwhile while durable selective coatings consistently reduce the LCOE-up to 12% of the value obtained for an uncoated receiver. The absolute LCOC is a powerful tool to characterize and compare different coatings, not only considering their initial efficiencies but also including their durability.

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.

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.

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