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DOE OSTI2024

Helium metastable density determination in the COST reference source by absolutely calibrated optical emission spectroscopy

Helium metastable densities in the COST Reference Microplasma Jet are estimated for a variety of He/N2 admixtures and dissipated powers by applying a collisional-radiative model to absolutely calibrated optical emission spectroscopy measurements. This is accomplished by delineating the excitation mechanisms that result in the N2(C–B) and N2+(B–X) emission bands, the latter of which is strongly coupled to the presence of helium metastables. A number of other plasma parameters are established and discussed for each operating condition including the electron energy distribution function, reduced electric field, rate constants, and electron density. With these parameters, the reaction rates for the primary ionization pathways are also calculated, emphasizing the importance of helium metastables for discharge sustainment. Good agreement with the existing literature is found for most plasma parameters and for helium metastable densities, in particular. A clear [N2]−1 relationship between the nitrogen concentration and density of helium metastables is demonstrated, as has been identified in previous studies in analogous atmospheric pressure plasma jets. This validates the efficacy of this optical technique for determining helium metastable densities and establishes it as a viable, and in many cases, more accessible alternative to other means of quantifying helium metastables in low-temperature plasmas.

Physics↗

DOE OSTI2025

Interactions of a Microdischarge with a Premixed Flame (Final Technical Report)

In this project we introduced an experimental platform that facilitates the study of the two-way interaction of plasma and flame, and the exploration of the impact of the spatial location of the discharge on its kinetics, in a controlled environment. The experimental platform is a mesoscale CH4/air premixed flame burner that also acts as a dielectric barrier discharge (DBD) operated by nanosecond pulsed voltage. A unique feature of this setup is that the DBD utilizes transparent electrodes, made of Indium Tin Oxide (ITO), to unlock optical access along the line-of-sight parallel to the dominant electric field. This platform is suitable to study both the forward problem (impact of plasma-on-flame), which manifests itself as a modified flame speed as well as flame wrinkling; and the backward problem (impact of flame-on-plasma), which includes plasma regime transitions, between microdischarges and uniform modes, driven by the flame passage. Prior work at MIT has focused on electrical measurements and direct imaging studies of the plasma-flame interaction through the ITO electrode. This project, which was awarded experimental runtime at the Low-temperature Plasma Research Facility (PRF) at Sandia National Laboratories, quantifies the flame-plasma interaction through temporally resolved measurement of key radicals (oxygen atom, O) produced by the nanosecond repetitively pused discharge (NRPD) in an individual microdischarge, using two-photon absorption laser-induced fluorescence (TALIF).

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2025

High Li + Transference Number Electrolyte Enabled by Fluoride Acceptor for Low-Temperature Li-Ion Batteries

To enable wide-temperature operation of lithium-ion batteries (LIBs), new electrolyte formulations have been developed to enhance the performance, particularly at low temperatures. A key challenge lies in achieving both high ionic conductivity and a high lithium-ion transference number due to their inherent trade-off. In this study, we designed an electrolyte system comprising tris(pentafluorophenyl)borane (TPFPB), a fluoride acceptor, and LiF salt in ethylene carbonate (EC)-free solvents. TPFPB, with its electron-deficient boron center, facilitates fluoride transfer reactions that promote the dissociation of otherwise insoluble LiF. When methyl acetate (MA) was used as the solvent, the electrolyte exhibited a high transference number (t Li + = 0.85) and ionic conductivity (σ = 5.0 × 10 –3 S cm –1 ). The optimized electrolyte demonstrated excellent performance at −20 °C, with no evidence of lithium plating. This work presents a new strategy for electrolyte design by leveraging cation desolvation to achieve high-performance LIBs for low-temperature and high-power applications.

anions↗

DOE OSTI2026

Plasma-Assisted Pre-Chamber Ignition System for Highly Dilute Stoichiometric Heavy-Duty Natural Gas Engines (Final Technical Report)

This project explored advanced ignition technologies to significantly enhance efficiency and reduce operating costs for heavy-duty natural gas engines operating at stoichiometric conditions, while meeting ultra-low NOx emission standards. The main goal was to develop and validate a plasma-assisted pre-chamber ignition system that could deliver at least a 2% increase in brake thermal efficiency (BTE) and a 4% decrease in total cost of ownership (TCO) compared to a typical multi-cylinder engine with three-way catalyst aftertreatment, ensuring compatibility with the expected 2027 EPA/CARB regulations. In the first half of the project, the research team concentrated on developing and testing plasma-assisted pre-chamber ignition using nanosecond pulsed discharges. Extensive experiments were conducted in an optically accessible rapid-compression and expansion machine, a constant-volume chamber, and an optical single-cylinder engine. Experiments were coupled with CFD simulations. The work produced unique insights into pre-chamber flame formation, jet ignition, dilution effects, and flame quenching at pressures, temperatures, and dilution levels relevant to engines. Although plasma-assisted ignition showed promise in controlled lab settings, the research also identified fundamental and practical challenges when applying this technology to real engine conditions. Midway through the project, a crucial pivot was made, guided by three key findings. First, the power electronics required for nanosecond plasma discharges were found to be too costly for commercial use, undermining the project’s cost-of-ownership goals. Second, nanosecond plasma ignition was highly sensitive to turbulent flow in the pre-chamber, resulting in lower ignition reliability than traditional spark under engine-like conditions. Third, achieving a truly diffuse low-temperature plasma at high pressures near top dead center was not possible, reducing the anticipated chemical enhancement benefits. These results collectively suggested that continuing with plasma-assisted ignition was unlikely to meet both efficiency and cost objectives. In response, the project shifted focus to a more realistic approach: enhancing traditional spark-based pre-chamber ignition with significantly less spark energy. Using insights gained earlier in the project, the team redesigned the pre-chamber to maintain high dilution tolerance and quick combustion, even with lower ignition energy. Testing confirmed that with optimized pre-chamber design and combustion timing, a lower-energy spark could reliably ignite highly diluted stoichiometric mixtures, reduce burn time, and boost thermal efficiency. Final engine testing and techno-economic analysis verified that this revised approach successfully achieved the project goals. The optimized pre-chamber ignition system provided over a 2% increase in calculated brake thermal efficiency compared to the baseline engine. Notably, the lower ignition energy and simplified hardware reduced component stress, extended maintenance intervals, and lowered the total cost of ownership. When used with stoichiometric operation and traditional three-way aftertreatment, the system remained compatible with near-zero NOx emissions targets without increasing cost or complexity in the emissions control system. In summary, although the project deviated from its initial plasma-assisted ignition idea, the work produced a more practical and commercially viable solution. The results show that precisely optimized, low-energy pre-chamber spark ignition can significantly improve efficiency and reduce overall ownership costs for heavy-duty natural gas engines. This directly aligns with DOE goals for cleaner, more efficient, and cost-effective transportation technologies.

03 NATURAL GAS↗

DOE OSTI2025

Flux Synthesis of Lattice‐Engineered Rutile Solid Solutions for Acidic Oxygen Evolution

Developing efficient and stable electrocatalysts for the acidic oxygen evolution reaction (OER) is vital for advancing proton exchange membrane water electrolysis (PEMWE) technologies. Here, in this study, we report a flux synthesis of nitrogen-doped Ti–Ru rutile-type solid-solution oxides (M-TiRu 4 ) using molten NaNO 3 as the flux medium. The flux medium promotes the low-temperature conversion of TiN to rutile TiO 2 , while in situ-formed RuO 2 nanoparticles facilitate lattice templating and couple with interfacial ion migration, enabling the formation of homogeneous solid solutions with abundant lattice heterogeneity. Simultaneously, nitrogen atoms are stably incorporated into the lattice of solid solutions, inducing bandgap narrowing, which enhances electronic conductivity. The developed M-TiRu 4 catalyst exhibits exceptional acidic OER performance, delivering a low overpotential of 194 mV at 10 mA cm −2 , superior durability over 600 h, and a Ru mass activity 7.8 times that of commercial RuO 2 . At the device level, M-TiRu 4 enables PEMWE operation at 1.64 V @ 2 A cm −2 and maintains stable performance at 500 mA cm −2 for 200 h with a minimal degradation rate of 20 µV h −1 . This work demonstrates a robust approach for designing high-performance, durable acidic OER catalysts via synergistic lattice and electronic structure engineering, paving the way for next-generation water-splitting technologies.

Wang, Fan [Univ. of Tennessee, Knoxville, TN (Unit↗

DOE OSTI2025

Tailoring Na⁺ Chelation Dynamics for Expedient Sulfur Redox Kinetics in Low‐Temperature Sodium–Sulfur Batteries

Sodium–sulfur (Na–S) batteries have attracted considerable attention due to their high theoretical energy density and the abundant natural availability of sodium and sulfur. However, sluggish kinetics of sulfur conversion, slow Na⁺ transport, and interfacial instability at low temperatures pose significant challenges for their operation and limit their practical application. Herein, three solvents with well-designed molecular configurations are examined. We systematically investigate the impact of chelation effect on the desolvation behavior, sulfur conversion process, ion dynamics, and Na⁺ plating/stripping behavior. Compared with conventional linear ether solvents, the incorporation of methyl groups not only weaken the chelation capability and tailors the inner solvation sheath, but also reduces the energy barrier for Na⁺ transport, thus promoting enhanced sulfur conversion kinetics under low temperature conditions. This work elucidates the relationship among solvent molecules, Na⁺ desolvation behavior, and sulfur reaction kinetics, and offers a strategy for rational design of electrolytes for low-temperature metal–sulfur batteries.

25 ENERGY STORAGE↗

DOE OSTI2025

Integration of a grey-box refrigerated case model in EnergyPlus via Python plugin

Commercial buildings, in particular grocery stores (due mainly to their large refrigeration load), provide opportunities for energy cost reductions. Grocery stores could offer substantial load flexibility to the power grid through participation in demand response programs because of their usage patterns and relatively high energy intensity. This load flexibility could come from modifying the control of heating, ventilation, and air conditioning (HVAC) systems, refrigeration systems, or both. Although estimation of the HVAC system’s load flexibility potential is relatively targeted in the literature, estimating load flexibility of refrigeration systems is nascent and has been a challenge, in part because of the lack of proper simulation tools that capture the dynamics in the refrigeration cases. The existing refrigerated case model within EnergyPlus, a whole building energy simulation program, assumes a constant case temperature throughout the simulation period and does not explicitly model the cycling of the compressor serving the refrigerated case. In addition, it does not encompass modeling of temperatures of the product inside the refrigerated case. This difference between modeled and actual operation can be a barrier to the development of demand control algorithm and accurate analysis of load flexibility potential. In this paper, we present a grey-box model for modeling refrigerated cases in grocery stores, which include medium temperature and low temperature. Four cases are modeled; two are low-temperature closed cases and two are medium-temperature cases with one closed and one open. Data from an experimental facility are used to train and test the models. Results demonstrate the efficacy of the grey-box models in predicting the temperatures. This model is integrated into EnergyPlus to capture the dynamic effects of case temperature on the environment and enhance the calculation of sensible and latent heat exchange with the environment (case credits). These enhancements can be leveraged more broadly to model advanced refrigeration controls such as defrost, develop and test unique algorithms that could affect refrigeration interactions with HVAC, and refine store design for any commercial building with refrigeration.

29 ENERGY PLANNING, POLICY, AND ECONOMY↗

DOE OSTI2025

Impact of gasoline composition on the effects of nitric oxide on autoignition and knock in a DISI engine

Modern spark-ignition engines use exhaust gas recirculation (EGR) to dilute the charge and suppress knock, enabling the use of higher compression ratios and/or more optimum combustion phasing for higher efficiency. The effectiveness of EGR is affected by the composition of the fuel and its chemical-kinetic interactions with combustion products. Among those, nitric oxide (NO) has been shown to strongly affect autoignition reactivity. However, the impact of fuel composition of the effect of NO on reactivity is not well-understood. Here, in this study, engine experiments were conducted to assess the impact of NO seeded to the intake on knock-limited operation of two gasoline fuels (high cycloalkane content, or HCA, and high olefin content, or HO). Results showed that compositionally-different fuels responded differently to NO. HCA, which was less knock-limited than HO for NO < 200 ppm, became more knock-limited for NO > 200 ppm. Moreover, it was found that differences in knock between fuels were caused by differences in autoignition chemistry and not in the sequential autoignition process of the end gas that occurs due to thermal stratification. Chemical kinetic simulations were performed to better understand the experimental results. For HCA, intermediate-temperature heat release had a greater impact on autoignition reactivity than low-temperature heat release, while the opposite was observed for HO. For both fuels, NO enhances the magnitude of low-temperature heat release via NO + HO 2 → NO 2 + OH. The effect of NO on reactivity was stronger for HCA because OH produced from NO helped to overcome the OH quenching effect of cyclopentane, a main species in HCA. In contrast, HO had relatively strong inherent low-temperature chemistry arising from iso-octane, which reduced the impact of NO on reactivity. For the range of NO mole fractions tested in this study, in-cylinder NO increased fuel’s knock propensity, especially for fuels with mild low-temperature chemistry.

Autoignition↗

DOE OSTI2025

Large area transparent refractory aerogels with high solar thermal performance

Application of transparent silica aerogels in low-temperature solar thermal systems has led to major improvements in performance. In high temperature concentrating solar thermal (CST) systems, aerogels have yet to demonstrate the necessary scalability, durability, and performance to support their widespread deployment. Here, large-area transparent refractory aerogel tiles are synthesized and shown to achieve a record-high receiver figure-of-merit (FOM) at high temperatures. The work leverages a scaled-up process for sol–gel synthesis to control the density of the aerogels for improved solar transmittance and adapts a previous atomic layer deposition (ALD) technique with the aid of predictive reaction-transport modeling. After aging for 10 days at 700 °C, the large-area tiles exhibit a solar-weighted transmittance of 95.6 % and a thermal emittance of 0.31, corresponding to a FOM of 80 % at 100 suns and 700 °C. The observed sintering rates at 700 °C are comparably low to earlier one-inch aerogels, suggesting long-term stability under relevant operating conditions. Furthermore, the study indicates that refractory aerogels are scalable materials for efficient photothermal conversion at high temperatures.

Aerogels↗

DOE OSTI2025

Surface Hydration of Porous Nickel Hydroxides Facilitates the Reversible Adsorption of CO 2 from Ambient Air

Direct air capture (DAC) under humid ambient conditions typically requires the use of organic components, with sorbents that are purely inorganic in composition for the most part operating hundreds of degrees above room temperature. In this work, we report porous metal hydroxides as a novel class of water-tolerant, oxidatively and hydrothermally stable low-temperature sorbents that exhibit competitive DAC working capacities of 1.25 mmol/g over 5 consecutive temperature swing adsorption–desorption cycles in the presence of steam and oxygen. Aqueous miscible organic solvent treatments are used to create highly porous structures with surface areas exceeding 700 m 2 /g that capture CO 2 in the form of bicarbonates under dry conditions, and carbonates under wet conditions. Water exerts a facilitative rather than an inhibiting effect on CO 2 binding, and the presence of hydrating multilayers serves to stabilize carbonate species akin to moisture swing adsorbents except for the fact that solvation results in a remarkable (upto 10-fold) increase, not decrease, in DAC capacity. High-valent doping with cerium is used to improve DAC capacities by amplifying surface basicity, evidencing porous nickel hydroxides specifically (and porous metal hydroxides more generally) as a novel class of robust, earth-abundant DAC sorbents.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Dynamic Cooling on Contemporary Quantum Computers

We study the problem of dynamic cooling whereby a target qubit is cooled at the expense of heating up N − 1 further identical qubits by means of a global unitary operation. A standard back-of-the-envelope high-temperature estimate establishes that the target qubit temperature can be dynamically cooled by at most a factor of 1 / N . Here we provide the exact expression for the minimum temperature to which the target qubit can be cooled and reveal that there is a crossover from the high initial temperature regime, where the scaling is 1 / N , to a low initial temperature regime, where a much faster scaling of 1 / N occurs. This slow, 1 / N scaling, which was relevant for early high-temperature NMR quantum computers, is the reason dynamic cooling was dismissed as ineffectual around 20 years ago; the fact that current low-temperature quantum computers fall in the fast, 1 / N scaling regime, reinstates the appeal of dynamic cooling today. We further show that the associated work cost of cooling is exponentially more advantageous in the low-temperature regime. We discuss the implementation of dynamic cooling in terms of quantum circuits and examine the effects of hardware noise. We successfully demonstrate dynamic cooling in a three-qubit system on a real quantum processor. Since the circuit size grows quickly with N , scaling dynamic cooling to larger systems on noisy devices poses a challenge. We therefore propose a suboptimal cooling algorithm, whereby relinquishing a small amount of cooling capability results in a drastically reduced circuit complexity, greatly facilitating the implementation of dynamic cooling on near-future quantum computers. Published by the American Physical Society2024

Physics↗

DOE OSTI2024

State of the art in low-temperature and high-temperature electrolysis

Water electrolysis is gaining traction in large-scale applications, with production of multiple technologies scaling to hundreds and thousands of megawatts of new electrolyzer capacity annually. Low-temperature electrolysis has dominated the electrolyzer market for decades, but still only represents a small amount of the overall hydrogen market, due to the higher production costs versus hydrogen derived from fossil fuels. Advances are needed in capital cost and efficiency to close the cost gap, especially for energy applications. Similarly, while high-temperature electrolyzers can operate more efficiently, reducing the operating cost, they still need further scale-up and cost reduction to compete in these markets. Understanding the recent advances in each and the priority research directions is important to focus and accelerate innovation, and will be discussed in this article. The different advantages and disadvantages of each of these technologies will also be reviewed; there will likely be applications for each in the overall deployment of renewable hydrogen.

36 MATERIALS SCIENCE↗

DOE OSTI2025

AUTOIGNITION DELAY TIMES FOR REFORMATE GAS MIXTURES FROM METHANE GAS ENGINES

Methane slip is a prominent issue in natural gas reciprocating engines that are used in transportation and marine applications. The incomplete combustion that results in methane slip can be resolved with the introduction of hydrogen within the combustion mixture to improve methane oxidation and further enable combustion within the engine crevices where methane has previously remained unreacted. Steam methane reforming (SMR) is a common method used to produce hydrogen and can be used to design an onboard device to reduce methane slip from reciprocating engines. The development of this reformer device requires the validation of high-fidelity chemical kinetic models at the low temperatures of the crevice volumes of these engines. In this work, auto-ignition data is obtained using a shock tube at lean (φ—0.714 or λ—1.4) and stoichiometric (φ, λ = 1) equivalence ratios spanning a temperature range of 1042–1234 K at the 80-bar operating pressure of the test engine. Blends of methane, hydrogen, and reformate products from the SMR reaction are shock-heated in synthetic air, with the ignition delay time measured using an OH* chemiluminescence detector at 310 nm and a CH* detector at 430 nm. The experimental results are compared to several state-of-the-art chemical kinetic mechanisms from the literature. In general, most of the mechanisms show very good agreement with experiments at higher temperatures, with simulation results showing little deviation from experiments at lower temperatures. A sensitivity analysis was conducted, and the results reveal that the reaction H2 + CH3O2 = H + CH3O2H has a very significant role in determining low-temperature ignition delay times (IDTs) of SMR mixtures. These findings provide valuable insights into the chemical kinetics governing methane reformate combustion and contribute to the optimization of onboard reformer designs aimed at mitigating methane slip in natural gas-fueled engines.

Fraze, Matthew↗

DOE OSTI2024

Generative Physics-Informed Neural Network Solving Multi-Scale and Multi-Phase Plasma Chemical Flow Field

Low-temperature plasmas (LTPs) are non-equilibrium systems with near-room-temperature gas and highly energetic electrons. This makes them ideal for delicate applications in biomedicine and semiconductor manufacturing, enabling processes like wound healing, sterilization, etching, and plasma-enhanced chemical vapor deposition without thermal damage. However, LTPs involve complex chemistries, with hundreds of species and thousands of reactions, complicating their diagnosis, prediction, and control. Conventional diagnostics, such as Fourier-transform infrared spectroscopy (FTIR), laser-induced fluorescence (LIF), and optical emission spectroscopy (OES), offer limited species detection, while mass spectrometry (MS) struggles with low-sensitivity species. Additionally, LTP simulations face multi-scale challenges, as macroscopic fluid dynamics and microscopic particle collisions operate on vastly different timescales. To address these issues, we developed an artificial intelligence (AI) based diagnostic system: a generative physics-informed neural network (PINN-Gen) that can predict spatially resolved species concentrations and temperatures in LTPs by integrating experimental data from planar LIF with microscopic plasma chemical kinetics and macroscopic fluid mechanics, including plasma-liquid interactions at the interface between two phases. PINN-Gen solves no equations but checks the errors of physical laws by substituting the output from neural network, and the comparison with the experimental results. Thus, it naturally avoids the multi-scale difficulty of numerical simulations and predicts the results of conventionally unsolvable multi-scale and multi-phase problems. The real-time prediction will be robust due to the physical information used in the training of such a neural network, and only very limited input of condition required due to its generative feature.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2026

Design and Validation of a Cryogenic Dilatometer

This project focuses on the design and development of a cryogenic dilatometer capable of measuring the thermal expansion of materials at extremely low temperatures. Understanding how materials change dimension with temperature is critical for applications involving cryogenic environments, particularly laminated systems housing superconducting magnets, where small dimensional changes can significantly affect performance and reliability. To address this, a dilatometer system was designed to operate within a liquid nitrogen environment while accurately measuring displacements caused by thermal contraction. The purpose of this device is to determine the coefficient of thermal expansion (CTE) of a material. The design process involved defining functional requirements, developing mechanical and thermal concepts for the measurement apparatus, selecting appropriate materials compatible with cryogenic temperatures, and integrating sensors capable of detecting small dimensional changes. The resulting system aims to provide a practical and repeatable method for evaluating material thermal expansion at cryogenic temperatures, supporting research into composite use in superconducting applications that require accurate characterization of material behavior in low-temperature environments.

Schmitt, Nicholas [Northern Illinois U.]↗

DOE OSTI2024

A Comparative Experimental Analysis of Natural Gas Dual Fuel Combustion Ignited by Diesel and Poly OxyMethylene Dimethyl Ether

Dual-fuel low-temperature combustion is a possible solution for alleviating the tradeoff between oxides of nitrogen and soot emissions in conventional diesel combustion, albeit with poor combustion stability, high carbon monoxide, and unburned hydrocarbon emissions at low engine loads. The present work compares emissions and combustion (heat release and other metrics) of both diesel and poly-oxy methylene dimethyl ether as high-reactivity fuels to ignite natural gas while leveraging spray-targeted reactivity stratification, which involved multiple injections of the high-reactivity fuels. The experiments included six parametric sweeps of: (1) start of first injection, (2) start of second injection, (3) percentage of energy substitution of natural gas, (4) commanded injection duration ratio, (5) rail pressure, and (6) intake pressure. The experiments were performed on a 1.8 L heavy-duty single-cylinder research engine operating at a medium speed of 1339 rev/min. Not-to-exceed limits for the indicated oxides of nitrogen emissions, maximum pressure rise rate, and the coefficient of variation of the indicated mean effective pressure were set to 1 g/kWh, 10 bar/CAD, and 10%, respectively. The indicated emissions decreased and combustion improved significantly for both fueling combinations when the experimental procedure was applied.

Partridge, Kendyl Ryan↗

DOE OSTI2024

A Planar-Cavity Receiver Configuration for High-Temperature Solar Thermal Processes: Preprint

Next generation concentrating solar thermal power (CSP) and novel solar thermochemical systems using concentrating solar thermal (CST) energy require high operating temperatures exceeding those of traditional nitrate-salt CSP systems. Particle-based systems are attractive for next-generation CSP and CST applications owing to high-temperature stability of inert silica- or alumina-based particulate materials, the lack of low-temperature freezing concerns that limit molten salt and/or molten metal heat transfer media, and cost-effective thermal storage using low-cost particulate and containment materials. Open-cavity falling particle receivers have many potential advantages, but face challenges pertaining to scalability, thermal loss, and particle loss through the open aperture, and are infeasible for chemical processes that require a low-oxygen ambient environment. Enclosed receiver configurations can be scalable, avoid particle loss when heating particles, and have potential for future chemical processes; however, particle-based heat transfer media provide substantially lower heat transfer rates than liquid media, and thus enclosed particle receiver designs require novel configurations to limit surface temperatures under the high incident solar flux concentrations necessary for high receiver thermal efficiency at high temperature. This paper introduces the novel planar-cavity enclosed particle receiver configuration in which arrays of planar surfaces are arranged into sub-vertical cavities. Large angles between the panel surface normal vectors and the aperture surface normal allow the incoming solar beam to distribute along the panel walls. Correspondingly, a high incident solar flux concentration at the cavity aperture produces substantially lower absorbed solar flux concentration on any panel wall. Sets of individual vertical cavities can be arranged to form a scalable receiver configuration.

cavity receiver↗

DOE OSTI2024

High-Fidelity and High-Performance Computational Simulations for Rapid Design Optimization of Sulfur Thermal Energy Storage

Industrial process heating (IPH) accounts for approximately 70% of US manufacturing energy use and is primarily produced by fossil fuel combustion. Approximately 1500 TWht (approximately 60%) of IPH demand is in the temperature range of 100-300. Industrial applications in this temperature range include drying, hydrothermal processing, thermal enhanced oil recovery, food and beverage, bioethanol production, etc. Cost-effective thermal energy storage (TES) that increases the utilization of waste and renewable heat (solar, geothermal, etc.) could provide significant energy savings and reliable heat sources, decrease emissions, and increase US manufacturing competitiveness through reductions in fuel consumption. TES development has historically been dominated by technologies suitable for deployment with concentrating solar power (CSP). State-of-the-art thermal storage deployed commercially with power tower CSP plants uses a 60%/40% NaNO3/KNO3 molten salt and operates between temperatures of approximately 280 degrees Celsius and 570 degrees Celsius using a two-tank configuration. However, these nitrate salts are unsuitable for operation outside of this temperature range due to a high freezing point of approximately 220 degrees Celsius, and limits on high-temperature salt stability and corrosion resistance of containment alloys. Other materials being investigated for TES include those based on: (1) sensible energy storage (various molten salt compositions, inert solid particles, rocks or pebble beds, sulfur, water, concrete, graphite, etc.), (2) latent energy storage in materials that undergo solid-liquid phase change at relevant temperatures (organic materials for low-temperature applications, inorganic salts and/or metals for high-temperature applications), or (3) thermochemical energy storage (hydrides, hydroxides, carbonates, metal oxides, etc.). The application temperature and challenges pertaining to storage material and/or containment cost, energy density, long-term thermal and cyclic stability, and charge/discharge heat transfer effectiveness drive material selection for a given IPH or electricity generation application. Sulfur is a cheap commodity at $80/ton compared to $1100 - 1300/ton for conventional salts. When using a metric of storage cost per kWh, sulfur costs around 2-3 $/kWh. Previous sulfur TES development focused on high temperature (>600 degrees) concentrated solar power applications with sulfur encapsulated in pipes and flow of gaseous HTF (air) in the shell side. However, for lower-temperature IPH applications in the range of approximately 100-300 degrees Celsius Element 16 adopted a compact and scalable TES design with molten sulfur in the shell and HTF pipes submerged in the molten sulfur bath. The low-cost molten sulfur TES for dispatchable IPH has deployment potential for broad applications. The spatial and temporal evolution of the HTF and sulfur temperature is critical to the TES system performance, and thus detailed modeling can improve understanding of the performance and facilitate design improvements. Using high performance computing and computational fluid dynamics (CFD) a low-cost molten sulfur thermal energy storage (TES) system for industrial process heating (IPH) applications was developed. The unique challenges in CFD modeling of sulfur TES are the sharp property changes of sulfur relevant to the working temperatures. Above 159, liquid sulfur undergoes polymerization, and the viscosity of sulfur rapidly increases by several orders of magnitude between 159 degrees Celsius and 188 degrees Celsius, followed by a decrease in viscosity beyond 188 degrees Celsius due to thermal bound dissociation. In addition, various concentrations of H2S impurities can also modify sulfur viscosity. This numerical challenge is especially relevant to transient simulation of the sulfur TES charging and discharging processes as the extreme property variations limit the applicability of traditional heat transfer correlations. Transient CFD simulations including the temperature-dependent sulfur properties and geometric complexity of the TES design were used to predict the effect of natural convection during charging and discharging on the heat transfer process, sulfur temperature uniformity, charge/discharge rates, and performance of the storage devices. The CFD model was validated with experimental results for a full charge and discharge cycle. The work will show 3D and 2D simulation comparisons aimed to facilitate rapid design iterations and a machine learning based design optimization approach.

CFD↗