Renkun Chen

ABSTRACT Thermal radiation is a major mode of heat transfer in high temperature porous materials such as insulations, ceramic foams, and thermal barrier coatings. However, quantification of radiation heat transfer is not straightforward due to the complexity of measuring optical parameters, and the coupling with conduction heat transfer. In this work, high‐temperature laser flash analysis measurements with a coupled radiation‐conduction model were used to directly quantify thermal radiation in monolithic porous samples of ceramic nanoparticles. These samples had sub‐50 nm pores stable up to 1000°C, thus suppressing solid and gaseous conduction, while minimizing infrared scattering. A combination of the resulting low thermal conductivity (<0.5 W m −1 K −1 ) and high transmittance of the samples leads to radiation dominant heat transfer at high temperatures. The contribution of radiative heat transfer was quantified using the Planck number (conduction‐to‐radiation parameter). Above 500°C, radiation heat transfer became significant and at higher temperatures, the contribution was up to five times larger than conduction.

Highly confined phonon polaritons enable strong light–matter interactions that tailor incandescent heat sources for enhanced thermal emission in both the near- and far-field regimes. However, single polar dielectric materials are limited in both the emission spectral range and achievable mode confinement. In this study, we employ a bilayer structure comprising monolayer hexagonal boron nitride (h-BN) integrated with silicon dioxide (SiO 2 ) to exploit confined phonon polariton modes across a broadened energy spectrum. The distinct, nonoverlapping Reststrahlen bands of h-BN and SiO 2 provide multiple spectral channels for polaritonic enhancement, improving far-field emission. We report a 3.4-fold enhancement in emissivity with the addition of h-BN to a SiO 2 nanoribbon. We identify the confined modes within the Reststrahlen bands with numerical modeling, revealing the enhancement mechanism. This effect is verified with direct thermal measurements by using a thermal bridge method, yielding a peak emissivity of 0.6. This work offers insights into engineering broad-band polaritonic thermal emitters.

Abstract General Engineering & Research (GE&R) has built an at-scale cryogenic magnetic refrigeration system and successfully demonstrated sustained sub 80 K (-193 °C) magnetocaloric cooling using a Halbach permanent magnet. The successful demonstration of cryogenic magnetic refrigeration using a permanent magnetic field with ZERO energy input requirements, validates this technology, and opens the door for its use in small and medium scale industrial applications, as well as fueling station infrastructure for fuel cell electric vehicles (FCEV). This paper provides an overview and status update of GE&R’s magnetic refrigeration technologies.

The development, operation, and regulation of nuclear reactors that utilize molten salts as fuel or as heat transfer media require knowledge of the thermal properties of the salt systems and quantification of the corresponding uncertainties. Knowledge of molten salt properties is also necessary for applications in material synthesis, processing, separations, solar thermal power generation, and energy storage. A round robin was conducted with national laboratory and university participants from twenty-one laboratories in five countries to compare property measurements, to better understand uncertainties, and to identify possible best practices. Two salt mixtures, each from a common batch, were distributed to participants for evaluation: equimolar NaCl-KCl and 45.0LiF-13.7NaF-41.3KF mol % (FLiNaK). Measurements were performed to determine the major constituent composition, oxygen content, density, thermal expansivity, melting point, and thermal conductivity. Error analysis was performed on each measurement for uncertainty quantification for each type of property that was explored. The resulting discussion of the methodologies used in this work is meant to lay the groundwork for the development of standard methods and reference materials for future high-temperature property measurements on halide melts.

We present systematic thermal conductivity (κ) measurements of suspended thin graphite ribbons, 234–527 nm thick, using a four-probe 3ω method. Unlike recent reports of phonon hydrodynamics and exceptionally high κ in micrometer-thick graphite ( Science, 2020 ), we observe significantly lower κ and no signatures of collective phonon flow in this intermediate thickness regime. Instead, our measured κ lies between few-layer graphene and bulk graphite. These results agree with a first-principles-informed Peierls–Boltzmann transport model with spatially resolved Monte Carlo sampling. Additionally, the temperature for the peak κ shifts lower with increasing thickness, due to the interplay of phonon-boundary and phonon-isotope scattering. Incorporating grain boundary scattering into simulations is necessary to replicate the experimental trends. These findings delineate the boundary between ballistic, hydrodynamic, and diffusive transport regimes in graphite and underscore the dominant role of disorder and geometry in phonon transport in quasi-two-dimensional materials, offering insights for nanoscale thermal management.

Extreme heat events with wet-bulb temperatures (WBT) above 35{\deg}C pose serious risks to human survival, and conventional hydrogel evaporative cooling alone may not provide sufficient relief as it must be maintained at a sufficiently high temperature to achieve effective evaporation in hot, humid conditions. This study integrates thermoelectric devices (TEDs) with hydrogels to create an effective personal cooling solution. TEDs pump heat away from the skin to maintain comfort while simultaneously increasing the temperature of hydrogel to enhance evaporation. This hybrid system outperforms TEDs or hydrogel alone in extreme conditions (temperature up to 55{\deg}C and relative humidity up to 88%, with WBT>35{\deg}C) and can operate for over six hours with a manageable hydrogel and battery weight. The active temperature control of TEDs allows adaptation to changing thermal loads and environments. These results demonstrate the potential of hybrid evaporative and thermoelectric cooling as an efficient, adaptable, and sustainable personal cooling solution to combat extreme heat.

Nanoneedles fabricated from refractory materials, such as copper cobaltate, are promising materials for solar energy conversion due to their favorable light-trapping properties at high temperatures. We demonstrate that coating these materials with a thin aluminum-doped zinc oxide (AZO) layer improves their optical properties dramatically, leading to a very low ( < 1 % ) reflectance in a wide spectral range, from the ultraviolet to the near-infrared ( 0 . 3 − 2 μ m). This advantageous property is present even at very large angles of incidence ( θ = 70 ° ), which makes this material attractive for increasing the acceptance angle of central-receiver concentrating solar power systems or as an ultra-black diffuse optical component for infrared imaging systems. Finally, the exceptionally high emissivity of this material in the near- and mid-infrared at temperatures up to 600 °C proves that its optical properties are thermally resistant and suggests that the material can be used as a high-temperature alternative to ultra-black coatings made of vertically aligned carbon nanotubes. • Ultra-black CuCo 2 O 4 coatings maintain high absorptance over a wide angular range. • AZO layer enhances emissivity above 0.9 up to 600 °C with thermal stability in air. • Bidirectional reflectance confirms broadband light absorption up to high angles. • AZO outperforms other oxides due to its combination of VIS and IR optical properties.

Thermoelectric Active Cooling Garments Extreme heat events are becoming more frequent, intense, and prolonged due to climate change. In article number 2401690, Renkun Chen and co-workers present a flexible, lightweight, and portable personal cooling garment powered by thermoelectric devices to help combat extreme heat. The garment is designed for comfort and wearability, providing sustained cooling even in ambient temperatures up to 40 °C.

Extreme heat events with wet-bulb temperatures (WBT) above 35°C pose serious risks to human survival, and conventional hydrogel evaporative cooling alone may not provide sufficient relief as it must be maintained at a sufficiently high temperature to achieve effective evaporation in hot, humid conditions. This study integrates thermoelectric devices (TEDs) with hydrogels to create an effective personal cooling solution. TEDs pump heat away from the skin to maintain comfort while simultaneously increasing the temperature of hydrogel to enhance evaporation. This hybrid system outperforms TEDs or hydrogel alone in extreme conditions (temperature up to 55°C and relative humidity up to 88%, with WBT &gt; 35°C) and can operate for over six hours with a manageable hydrogel and battery weight. The active temperature control of TEDs allows adaptation to changing thermal loads and environments. These results demonstrate the potential of hybrid evaporative and thermoelectric cooling as an efficient, adaptable, and sustainable personal cooling solution to combat extreme heat.

800 °C) in ambient air in a porous solid thermal insulation material, using stable packed nanoparticles of high-entropy spinel oxide with 8 cations (HESO-8 NPs) with a relatively high packing density of ≈50%, is reported. The high-density HESO-8 NP pellets possess around 1000-fold lower thermal diffusivity than that of air, resulting in much slower heat propagation when subjected to a transient heat flux. The low thermal conductivity and diffusivity are realized by suppressing all three modes of heat transfer, namely solid conduction, gas conduction, and thermal radiation, via stable nanoconstriction and infrared-absorbing nature of the HESO-8 NPs, which are enabled by remarkable microstructural stability against coarsening at high temperatures due to the high entropy. This work can elucidate the design of the next-generation high-temperature thermal insulation materials using high-entropy ceramic nanostructures.

The tenth U.S.–Japan Joint Seminar on Nanoscale Transport Phenomena was held in San Diego, California, from July 16–19, 2023. The goals of the joint seminar series, established in 1993, are to encourage research and international exchange between US and Japan researchers in the nanoscale thermal transport community, foster US-Japan collaborations, and expose new junior scientists to leading-edge research in an interdisciplinary and international environment. The research topics were organized into 8 topical sessions, including (1) and (2) Conduction; (3) radiation and photonics; (4) and (5) Applications/Devices; (6) Fluids/Phase change; (7) Magnetism/Phonons; and (8) Thermal transport. The joint seminar opened with a plenary session and additionally featured an expert industry panel which discussed the industrial applications of thermal transport phenomena. An evening poster session provided graduate students and postdoctoral scholars with the opportunity to present their latest research results. A total of 99 researchers participated, with 51 from the United States and 48 from Japan. Of these participants, 47 were faculty, 9 held positions at national laboratories, industry, or government, and 43 were students or postdocs. The meeting was organized by Renkun Chen, Gota Kikugawa, Austin J. Minnich, and Junichiro Shiomi. Around 16 of the participants served as session chairs. The summaries of the various sessions prepared by the organizers and session chairs are presented in this report.

Abstract With the increasing frequency, intensity, and duration of extreme heat events due to climate change, heat‐related diseases or even mortality have become more prevalent. An efficient personal cooling strategy can mitigate heat stress by regulating the skin temperature within the thermal comfort zone. However, lightweight, wearable, and sustainable cooling garments are unavailable today. Here, the study develops a thermoelectric device (TED)‐based cooling garment and demonstrates its effectiveness in active personal cooling. The garment is shown to maintain the skin temperature within its thermal comfort zone in a hot environment of up to 40 °C under mild forced convection conditions (air flow speed of 2.2 m s −1 ). Furthermore, the study demonstrates a portable cooling system with less than 700 grams of total weight, which includes the TED‐based garment, a battery pack, and a temperature controller. The system showed long‐term cooling on the skin with varying ambient temperatures from 35 to 40 °C. With the advantages of lightweight, flexible, controllable, and long‐term effective cooling, the TED cooling garments described in this work can contribute to enhanced health and comfort in an increasingly hotter climate.

The project aimed to develop modulated photothermal radiometry (MPR) for conducting in-operando measurements of thermal transport in moving particle bed heat exchangers (MPBE) located at Sandia National Laboratory (SNL). The plan was to initially conduct lab-scale measurements on a 1-meter-long particle channel at UCSD, followed by in- operando measurements at SNL on a 20 kWth MPBE prototype built for this project, and if time permitting, potential final deployment on SNL’s 1 MWth G3P3 MPBE. The first two goals were successfully achieved but due to changes with commissioning timeline of the G3P3 MPBE, in-operando measurements on the G3P3 MPBE could not be conducted. The measurement campaigns have resulted in some promising results and the MPR tool has been handed over to trained SNL personnel for future use. MPR measurements on UCSD’s 1-meter-long particle channel showed a peculiar length dependence of the particle bed thermal conductivity and near-wall thermal resistance. The bed thermal conductivity increased, and the near-wall thermal resistance decreased, with increasing distance from the channel inlet, both eventually reaching a plateau. This was found to be due to hydrostatic pressure screening by the channel walls, known as the “Janssen effect”, and is the first experimental study on its role in granular heat transfer. A 20 kWth small-scale MPBE prototype, the first of its kind with 3 mm particle channels and 2 mm sCO2 plates, was designed and manufactured by Vacuum Process Engineering (VPE) for this project. This MPBE prototype was installed in SNL’s HEX test stand and the MPR tool was integrated into the test stand for in-operando measurements. Several modifications have been made to the MPR setup for facile installation and easy operation at SNL. Measurements with Carbo HSP40/70 particles were conducted with various particle and sCO2 temperatures and flow rates. Further, apart from regular MPBE operation, fault scenarios such as plate warping, particle stagnation, and choking were also detected and MPR showed excellent sensitivity to the detection of such conditions. Through the course of this project, MPR has evolved from a sensitive laboratory technique to a rugged industrial tool with various applications in high temperature measurements and diagnostics.

With the increasing frequency, intensity, and duration of extreme heat events due to climate change, heat-related diseases or even mortality have become more prevalent. An efficient personal cooling strategy can mitigate heat stress by regulating the skin temperature within the thermal comfort zone. However, lightweight, wearable, and sustainable cooling garments are unavailable today. Here, we developed a TED-based cooling garment and demonstrated its effectiveness in active personal cooling. The garment is shown to maintain the skin temperature within its thermal comfort zone in a hot environment of up to 40 oC under mild forced convection conditions (air flow speed of 2.2 m s-1). Furthermore, we demonstrated a portable cooling system with less than 700 grams of total weight, which includes the TED-based garment, a battery pack, and a temperature controller. The system showed long-term cooling on the skin with varying ambient temperatures from 35 to 40 oC. With the advantages of lightweight, flexible, controllable and long-term effective cooling, the TED cooling garments described in this work can contribute to enhanced health and comfort in an increasingly hotter climate.

Abstract In concentrated solar power (CSP) applications, fluidized bed is a promising approach for high heat transfer coefficient (HTC) solar receivers and heat exchangers. However, the complexity of multiphase mixing has made it difficult to characterize and analyze the heat transfer mechanism. This paper presents an experimental study on simultaneously characterizing heat transfer in both the near-wall and the bulk regions of a fluidized bed using modulated photothermal radiometry (MPR). The MPR is a non-contact frequency-domain technique using an intensity-modulated laser as the heat source and surface infrared emission as thermometry. The thermal penetration depth of the laser heating is varied by controlling its modulation frequency, and thus the measurement can resolve the near-wall and the bulk thermal resistances. With the MPR technique, we measured fluidized silica sands with a mean size of 164 μm in a vertical channel of 6 mm depth. Our results show that the near-wall thermal resistance is substantially increased with increasing gas velocity, which partially offsets the benefit of higher HTC brought by stronger particle mixing during the fluidization. We also used the MPR to quantify the improvement in particle-wall heat transfer in an inclined channel. We found that an 8° inclination towards the heat exchanging side led to a lower near-wall thermal resistance and a higher HTC at high gas velocities. This work demonstrates that the MPR technique is a useful tool to quantify the important near-wall thermal resistance from a bulk particle bed, which not only advances our understanding of heat transfer in fluidized beds, but may also contribute to the design of fluidized bed heat exchangers with higher HTC.

Abstract Radiative cooling has been recently intensively explored for thermal management and enhancing energy efficiency. Yet, traditional materials with singular emissivity fall short in dynamic thermal management, highlighting the need for materials that can adjust their thermal radiation in real time. Active modulation methods, requiring external stimuli such as mechanical stretch, electric potential, or humidity change, offer adaptability but can increase energy use and complexity. Passive approaches, using materials' inherent thermal‐responsive properties, face manufacturing and scalability challenges. Here, a scalable yet effective passive approach is introduced for adaptive thermal modulation based on gold (Au) and liquid crystal elastomer (LCE) with a reversible response to environmental temperature changes. This modulator enables a “low thermal resistance” state through actuation‐induced microcracks that expose a high‐emissivity polymer substrate, and a “high thermal resistance” state by closing these microcracks and forming a high thermal resistance air gap between the modulator and the target object. The flexible design and fixed external dimensions of the Au‐LCE thermal modulator make it adaptable to various surface geometries. Furthermore, by adjusting the LCE's chemical composition, the modulator's transition temperature can be tailored, broadening its applications from enhancing building energy efficiency to improving clothing thermal comfort.

Utilizing thermochemical reactions for thermal energy storage and solar fuel production has been an emerging research topic. Thermal transport properties of the materials are an important parameter that can determine the kinetics and efficiency of thermochemical reactions. With the increasing number of new thermochemical materials (TCMs); however, there is a lack of reliable techniques to monitor the thermal transport property of the materials and their changes as a function of reactions in real time. In this work, we report the in situ monitoring of thermochemical reactions using modulated photothermal radiometry (MPR). The thermal conductivities of two TCMs, namely, calcium hydroxide (Ca(OH)2) and Ba0.15Sr0.85FeO3−δ (BSF1585), were measured as a function of temperature and time using the MPR technique. The measured thermal conductivities were correlated to the reaction. The work has two significant contributions to the research communities. First, it provides a non-invasive diagnostic tool for monitoring the thermal transport properties of TCMs that can potentially be a high-throughput measurement technique conducive to optimizing TCMs, reactors, and related thermal systems. Second, for TCMs that show observable changes in thermal transport properties, a correlation between the measured thermal conductivity and the conversion fraction of the reaction can be established for monitoring the reaction kinetics based on thermal characterization.

A new series of 20-component fluorite-based compositionally complex oxides (20CCFBOxNb/Ta) with the general chemical formula (15RE1/15)2x+1(Ce1/3Zr1/3Hf1/3)3-3x(Nb1/2Ta1/2)xO8-δ (0 ≤ x ≤ 1, where 15RE1/15 = La1/15Pr1/15Nd1/15Sm1/15Eu1/15Gd1/15Tb1/15Dy1/15Y1/15Ho1/15Er1/15Tm1/15Yb1/15Lu1/15Sc1/15) are synthesized. Despite that the Gibbs phase rule allows for the existence of up to 20 phases at the thermodynamic equilibrium, 17 of the 20CCFBOxNb/Ta compositions synthesized in this study all possess single ultrahigh-entropy phases in fluorite, pyrochlore, or weberite structure, as shown by X-ray diffraction (XRD). Only < 1 vol.% of secondary phases are observed in two compositions near the phase-transition points. With changing compositional variable x, this series of 20CCFBOxNb/Ta undergoes an abrupt fluorite-pyrochlore transition at x = ~0.27 and an abrupt pyrochlore-weberite transition at x = ~0.87. Careful characterization reveals abrupt changes of order parameters at both phase transitions. In addition, weberite short-range ordering can persist into the long-range pyrochlore phase, which leads to the lowest thermal conductivities.

Sensations of heat and touch produced by receptors in the skin are of essential importance for perceptions of the physical environment, with a particularly powerful role in interpersonal interactions. Advances in technologies for replicating these sensations in a programmable manner have the potential not only to enhance virtual/augmented reality environments but they also hold promise in medical applications for individuals with amputations or impaired sensory function. Engineering challenges are in achieving interfaces with precise spatial resolution, power-efficient operation, wide dynamic range, and fast temporal responses in both thermal and in physical modulation, with forms that can extend over large regions of the body. This paper introduces a wireless, skin-compatible interface for thermo-haptic modulation designed to address some of these challenges, with the ability to deliver programmable patterns of enhanced vibrational displacement and high-speed thermal stimulation. Experimental and computational investigations quantify the thermal and mechanical efficiency of a vertically stacked design layout in the thermo-haptic stimulators that also supports real-time, closed-loop control mechanisms. The platform is effective in conveying thermal and physical information through the skin, as demonstrated in the control of robotic prosthetics and in interactions with pressure/temperature-sensitive touch displays.

Heat transfer across a granular flow is comprised of two resistances in series : near the wall and within the bulk particle bed, neither of which is well understood due to the lack of experimental probes to separate their respective contribution. Here, we use a frequency modulated photothermal technique to separately quantify the thermal resistances in the near-wall and the bulk bed regions of particles in flowing states. Compared to the stationary state, the flowing leads to a higher near-wall resistance and a lower thermal conductivity of bulk beds. Coupled with discrete element method simulation, we show that the near-wall resistance can be explained by particle diffusion in granular flows.

Determination of heat transfer coefficients for flowing packed particle beds is essential to the design of particle heat exchangers, and other thermal processes. While such dense granular flows fall into the well-known plug-flow regime, the discrete nature of granular materials alters the thermal transport processes in both the near-wall and bulk regions of flowing particle beds from their stationary counterparts. As a result, heat transfer correlations based on the stationary particle bed thermal conductivity could be inadequate for flowing particles in a heat exchanger. Earlier works have achieved reasonable agreement with experiments by treating granular media as a plug-flow continuum with a near-wall thermal resistance in series. However, the properties of the continuum were often obtained from measurements on stationary beds owing to the difficulty of flowing bed measurements. In this work, it was found that the properties of a stationary bed are highly sensitive to the method of particle packing and there is a decrease in the particle bed thermal conductivity and increase in the near-wall thermal resistance, measured as an effective air gap thickness, on the onset of particle flow. These variations in the thermophysical properties of stationary and flowing particle beds can lead to errors in heat transfer coefficient calculations. Therefore, the heat transfer coefficients for granular flows were calculated using experimentally determined flowing particle bed thermal conductivity and near-wall air gap for ceramic particles -CARBOCP40/100(275 um), HSP40/70(404um) and HSP16/30(956um); at velocities of 5-15mms-1; and temperatures of 300-650C. The thermal conductivity and air gap values for CP40/100 and HSP40/70 were further used to calculate heat transfer coefficients across different particle bed temperatures and velocities for different parallel-plate heat exchanger dimensions.

With the increasing miniaturization and power of optoelectronic devices, direct bonding of optical substrates like semiconductors and ceramics to metal heat sinks using low melting-point solder has gained significant interest. In this study, we demonstrated the bonding of glass to copper using Sn-58 wt% Bi solder (SB solder) doped with a small amount of rare earth (RE) elements. The RE elements act as active agents that facilitate the bonding to glasses without glass metallization. By optimizing the bonding parameters, such as reflow temperature and time, and employing an inert gas atmosphere to prevent solder or RE oxidation, we successfully achieved the highest shear strength in glass-copper solder joints using SB-RE solder, without the need for ultrasonic-assisted soldering (UAS). These results demonstrate the potential of using RE-containing solder for bonding unmetallized glass and ceramics in optoelectronic devices with metals at low soldering temperatures (< 200 °C). Furthermore, analysis of the shear strength and failure morphology of solder joints revealed only small degradation, primarily originating from the bulk solder region rather than the solder-glass interface, after both thermal aging (100 h) and cycling tests (100 cycles). The establishment of low-melting point RE-containing solders opens the possibility of direct jointing ceramic optoelectronic substrates to metal heat sinks for more efficient heat dissipation. In the meantime, our work also suggests that further optimization studies are necessary to explore its performance under more extreme working conditions.

Abstract Heat conduction in solids is typically governed by the Fourier’s law describing a diffusion process due to the short wavelength and mean free path for phonons and electrons. Surface phonon polaritons couple thermal photons and optical phonons at the surface of polar dielectrics, possessing much longer wavelength and propagation length, representing an excellent candidate to support extraordinary heat transfer. Here, we realize clear observation of thermal conductivity mediated by surface phonon polaritons in SiO 2 nanoribbon waveguides of 20-50 nm thick and 1-10 μm wide and also show non-Fourier behavior in over 50-100 μm distance at room and high temperature. This is enabled by rational design of the waveguide to control the mode size of the surface phonon polaritons and its efficient coupling to thermal reservoirs. Our work laid the foundation for manipulating heat conduction beyond the traditional limit via surface phonon polaritons waves in solids.

Heat transfer to a granular flow is comprised of two resistances in series: near the wall and within the bulk particle bed, neither of which is well understood due to the lack of experimental probes to separate their respective contribution. Here, we use a frequency modulated photothermal technique to separately quantify the thermal resistances in the near-wall and the bulk bed regions of particles in flowing states. Compared to the stationary state, the flowing leads to a higher near-wall resistance and a lower thermal conductivity of bulk beds. Coupled with discrete element method simulation, we show that the near-wall resistance can be explained by particle diffusion in granular flows.

Molten salts are a leading candidate for high-temperature heat transfer fluids (HTFs) for thermal energy storage and conversion systems in concentrated solar power (CSP) and nuclear energy power plants. The ability to probe molten salt thermal transport properties in both stationary and flowing status is important for the evaluation of their heat transfer performance under realistic operational conditions, including the temperature range and potential degradation due to corrosion and contamination. However, accurate thermal transport properties are usually challenging to obtain even for stagnant molten salts due to different sources of errors from convection, radiation, and corrosion, let alone flowing ones. To the best of authors' knowledge, there is no available in-situ technique for measuring flowing molten salt thermal conductivity. Here, we report the first in-situ flowing molten salt thermal conductivity measurement using modulated photothermal radiometry (MPR). We could successfully perform the first in-situ thermal conductivity measurement of flowing molten $NaCl-KCl-MgCl_2$ in the typical operating temperature (520 and 580 $^oC$) with flow velocities ranging from around 0.3 to 1.0 $m$$s^-1$. The relative change of the molten salt thermal conductivity was measured. Gnielinski's correlation was also used to estimate the heat transfer coefficient h of the flowing $NaCl-KCl-MgCl_2$ in the given experimental condition. The work showed the potential of the MPR technique serving as an in-situ diagnostics tool to evaluate the heat transfer performance of flowing molten salts and other high-temperature HTFs.

Molten salts are being used or explored for thermal energy storage and conversion systems in concentrating solar power and nuclear power plants. Thermal conductivity of molten salts is an important thermophysical property dictating the performance and cost of these systems, but its accurate measurement has been challenging, as evidenced by wide scattering of existing data in literature. The corrosive and conducting nature of these fluids also leads to time consuming sample preparation processes of many contact-based measurements. Here, we report the measurement of thermal conductivity of molten salts using a modulated photothermal radiometry (MPR) technique, which is a laser-based, non-contact, frequency-domain method adopted for molten salts for the first time. By unitizing the advantages of front side sensing of frequency-domain measurements and the vertical holder orientation, the technique can minimize the natural convection and salt creeping effects, thus yielding accurate molten salt thermal conductivity. The MPR technique is first calibrated using standard molten materials including paraffin wax and sulfur. It is then applied on measuring pure nitrate salts ($NaNO_3$ and $KNO_3$), solar salt ($NaNO_3-KNO_3$ mixture), and chloride salt ($NaCl-KCl-MgCl_2$). The measurement results are compared with data from literature, especially those obtained from laser flash analysis (LFA). Our results demonstrate that the MPR is a convenient and reliable technique of measuring thermal conductivity of molten salts. Accurate thermal conductivity data of molten salts will be valuable in developing the next-generation high-temperature thermal energy storage and conversion systems.

Molten salts are a leading candidate for high-temperature heat transfer fluids (HTFs) for thermal energy storage and conversion systems in concentrated solar power (CSP) and nuclear energy power plants. The ability to probe molten salt thermal transport properties in both stationary and flowing status is important for the evaluation of their heat transfer performance under realistic operational conditions, including the temperature range and potential degradation due to corrosion and contamination. However, accurate thermal transport properties are usually challenging to obtain even for stagnant molten salts due to different sources of errors from convection, radiation, and corrosion, let alone flowing ones. To the best of authors' knowledge, there is no available in-situ technique for measuring flowing molten salt thermal conductivity. Here, we report the first in-situ flowing molten salt thermal conductivity measurement using modulated photothermal radiometry (MPR). We could successfully perform the first in-situ thermal conductivity measurement of flowing molten $NaCl-KCl-MgCl_2$ in the typical operating temperature (520 and 580 $^oC$) with flow velocities ranging from around 0.3 to 1.0 $m$$s^-1$. The relative change of the molten salt thermal conductivity was measured. Gnielinski's correlation was also used to estimate the heat transfer coefficient h of the flowing $NaCl-KCl-MgCl_2$ in the given experimental condition. The work showed the potential of the MPR technique serving as an in-situ diagnostics tool to evaluate the heat transfer performance of flowing molten salts and other high-temperature HTFs.

Molten salts are being used or explored for thermal energy storage and conversion systems in concentrating solar power and nuclear power plants. Thermal conductivity of molten salts is an important thermophysical property dictating the performance and cost of these systems, but its accurate measurement has been challenging, as evidenced by wide scattering of existing data in literature. The corrosive and conducting nature of these fluids also leads to time consuming sample preparation processes of many contact-based measurements. Here, we report the measurement of thermal conductivity of molten salts using a modulated photothermal radiometry (MPR) technique, which is a laser-based, non-contact, frequency-domain method adopted for molten salts for the first time. By unitizing the advantages of front side sensing of frequency-domain measurements and the vertical holder orientation, the technique can minimize the natural convection and salt creeping effects, thus yielding accurate molten salt thermal conductivity. The MPR technique is first calibrated using standard molten materials including paraffin wax and sulfur. It is then applied on measuring pure nitrate salts ($NaNO_3$ and $KNO_3$), solar salt ($NaNO_3-KNO_3$ mixture), and chloride salt ($NaCl-KCl-MgCl_2$). The measurement results are compared with data from literature, especially those obtained from laser flash analysis (LFA). Our results demonstrate that the MPR is a convenient and reliable technique of measuring thermal conductivity of molten salts. Accurate thermal conductivity data of molten salts will be valuable in developing the next-generation high-temperature thermal energy storage and conversion systems.

Passive cooling of high-power electronics with minimum energy and water input is critical for the global water-energy nexus but has been challenging because of the large fluctuation in power and heat loads between the on/off-peak hours. Here we develop a moisture thermal battery (MTB) by coating superabsorbent hydrogel onto a pin fin heat sink with a large surface. The MTB leverages large latent heat and high thermal conductance of water evaporation for electronics cooling during on-peak hours and, importantly, autonomously harvests atmospheric moisture and stores the water during off hours. The MTB provides a thermal capacity of up to ∼200 kWh m−2 for evaporative cooling with a record-high effective heat transfer coefficient of up to ∼1,000 W m−2 K−1 for a passive device (no external power). The MTB suppresses the temperature fluctuation of a high-power field effect transistor and a computer CPU between the on- and off-peak hours by ∼15 K.