Transient Transport Processes Research Articles

Analysis of GAAFET’s transient heat transport process based on phonon hydrodynamic equations

Compared to the classical Fourier’s law, the phonon hydrodynamic model has demonstrated significant advantages in describing ultrafast phonon heat transport at the nanoscale. The Gate-All-Around FieldEffect Transistor (GAAFET) greatly optimizes its electrical performance through its three-dimensional channel design, but its nanoscale characteristics also lead to challenges such as self-heating and localized overheating. Therefore, it is of great significance to study the internal heat transport mechanism of GAAFET devices to obtain the thermal process and heat distribution characteristics. Based on this, this paper conducts theoretical and numerical simulation analyses on the phonon heat transfer characteristics within nanoscale GAAFET devices. Firstly, based on the phonon Boltzmann equation, the phonon hydrodynamic model and boundary conditions are rigorously derived, establishing a numerical solution method based on finite elements. For the novel GAAFET devices, the effects of factors such as surface roughness, channel length, channel radius, gate dielectric, and interface thermal resistance on their heat transfer characteristics are analyzed. The research results indicate that the larger the surface roughness, the smaller the channel length and the channel radius, the larger the interface thermal resistance leads to the higher hot spot peak temperature. The non-Fourier heat analysis method based on the phonon hydrodynamic model and temperature jump condition within the continuous medium framework constructed in this paper can accurately predict the non-Fourier phonon heat conduction process inside GAAFET and reveal the mechanisms of resistive scattering and phonon/interface scattering. This work provides important theoretical support for further optimizing the thermal reliability design of GAAFET, improving its thermal stability, and operational performance.

Redox-zoning in high-energy subterranean estuaries as a function of storm floods, temperatures, seasonal groundwater recharge and morphodynamics

Redox conditions are a major control for the concentrations and mass fluxes of water constituents, e.g., nutrients, metals and organic molecules within subterranean estuaries (STE). Due to the transient flow and transport processes in STEs as well as the variable temperatures and input of redox reactants, redox zoning in the STE is believed to be highly dynamic. In the present study we analyzed the individual and combined effects of storm floods, seasonal changes of temperatures and groundwater recharge rates, as well as beach morphodynamics on the redox zoning in the STE. For this purpose, a 2D cross-shore density-dependent flow and reactive transport model was set up representing a beach aquifer exposed to high-mesotidal and medium-to high-energy wave conditions, as well as to storm floods. The simulation results show that under the given conditions, redox dynamics may occur down to a depth of 20 m. Morphodynamics appear to be the most important factor for the transience of redox zones compared to the other factors. Seasonal changes in meteoric groundwater recharge rates appear to be least relevant for the redox dynamics in STEs.

Comprehensive analysis of the contribution from the heat capacity in thermal measurement on nanoscale thin films

Nanofilms (NFs) have been widely used in many emerging applications, such as microelectronic devices, spintronics, and optical instruments. Characterizing the thermal conductivity (k) of NFs, kNFs, is nontrivial for both fundamental science and industrial applications. Time-domain thermoreflectance (TDTR) is a powerful technique for thermal characterizations under nano-to-micro-scales. However, both the kNFs and the interface thermal resistance between layers are generally unknown parameters when analyzing TDTR signals. So that an effective thermal resistance model (ETRM) is often utilized, where the impact of heat capacity of NFs, cNFs, has been always ignored. Previous studies have proposed viewpoints on how to validate this assumption, however, which still needs to be verified further by considering the parameters' sensitivities for TDTR signals. In this work, we have highlighted the significance of sensitivity analysis for the investigation of the impact of cNFs with the example experiments on Ni NFs. The error of ETRM has been quantitatively and systematically studied. We found that the application requirements of ETRM in TDTR data analysis rely on the sensitivity relations between kNFs and interface thermal resistance. Finally, suggestions for future characterization of the kNFs have been discussed for reference. Our results and conclusions deepen the understanding of both the transient thermal transport process of multilayer NFs and the ability of TDTR on characterizing kNFs.

Analysis of the impact of carbon dissolution and energy transport on the flow in the hearth of an ironmaking blast furnace by transient CFD simulations

In the ironmaking blast furnace, layers of coke, ferrous burden (sinter, pellets, lump ore) and additions are heated by an uprising gas flow. While the layers are descending, the iron ore is reduced and begins to melt. The liquid products slag and hot metal trickle downwards into the hearth, where they prevail as two immiscible phases filling the voids within the coke bed and with the slag floating on top of the hot metal. As the hearth is lined with carbon blocks, which are eroded by the passing hot metal flow, this region has a major impact on the campaign length of the blast furnace. The blast furnace hearth is drained intermittently by opening the taphole and closing it to end the tap. Hence, the fluid phases in the hearth are in a transient state with the liquid levels constantly varying and the flow conditions being determined by the tapping/non-tapping state. In addition, dissolution of carbon into the hot metal and cooling applied to the refractory affect the local hot metal flow. This work presents a CFD-model, which is capable to calculate the multiphase flow in a 3-dimensional hearth at an industrial scale. The tool allows for modelling of the transient transport processes, including multiple tapping cycles by utilizing new approaches for the transition between tapping/non-tapping and the inflow of liquids into the hearth. Furthermore, the dissolution of carbon, energy transport in the liquids and the locally varying density of hot metal depending on temperature and carbon content are included. With this approach, thermal stratification and circulation effects induced by buoyancy can be described over a period of nearly 8 h, thus covering 4 tapping cycles. In addition to an analysis of energy, carbon content and velocity fields, the transient evolution of tapped hot metal properties (temperature, carbon) are obtained and compared to available plant data.

A compact jet array impingement cooling system driven by integrated piezoelectric micropump

Thermal management has become a bottleneck for high-power electronics due to high heat flux. Liquid cooling has been developed as an effective way to dissipate the heat of electronic devices. However, the existing liquid cooling systems are mostly too bulky to be applied in the miniaturized devices with very limited installation space. In this work, by integrating jet impingement cooling with piezoelectric micropump, we proposed a compact liquid cooling system that simultaneously enables high heat removal capability and tiny volume. The functions of jet array impingement cooling and coolant pumping are realized in a single component with an external dimension of 40 mm × 40 mm × 10 mm. The thermal performance of the system was investigated by experiments. The results show that the convective heat transfer coefficient achieves 20572 W·m−2·K−1 at a heat flux of 91.5 W/cm2. And the electric power consumption of the system is only 0.023 W under a heat load of 200 W (50 W/cm2). Simulations, which agree well with experiments, were further employed to study the transient heat transport process. This novel jet impingement cooling system driven by integrated piezoelectric micropump is compact and efficient, which provides a promising solution for the thermal management of high-power, miniaturized electronic devices.

A numerical assessment on the managed aquifer recharge to achieve sustainable groundwater development in Chaobai River area, Beijing, China

Intensive groundwater exploitation has depleted groundwater storage and led to a series of geo-environmental problems in Beijing Plain, China. Managed Aquifer Recharge (MAR) has been endorsed to mitigate the groundwater storage depletion and achieve groundwater sustainability. A pilot MAR has been tested in the Chaobai River catchment since 2015. An innovative large-scale MAR consisting of 9 cascade terraced infiltration ponds was proposed and its effectiveness was assessed in this study using an integrated modelling approach. The integrated model coupled the regional and local transient flow and transport processes. The transient regional flow model simulated historical groundwater level declines and storage depletion in the Beijing Plain from 1995 to 2018. The coupled regional and local flow model was used to simulate the pilot MAR test in the Chaobai River from 2015 to 2018. A significant groundwater level increase was observed nearby the pilot MAR since 2015. The transport model results indicate that approximately 40% of the infiltrated water was captured by pumping wells in the No.8 well field. The models were further used to assess the long-term effects of the large-scale MAR from 2020 to 2050. The simulation results show that the groundwater system will reach a new equilibrium state under the implementation of the large-scale MAR scheme. Almost 91% of the abstracted water in the No. 8 well field will come from the MAR infiltration. The proposed large-scale MAR is very effective in restoring the depleted aquifer storage and maintaining the groundwater abstraction in the No.8 well field. However, with the increase of the groundwater level, the infiltration rate of several ponds will decrease. Therefore, it is important to maintain a dynamic balance between artificial recharge and groundwater abstraction in order to achieve a sustainable long-term MAR operation in the region.

Model-based optimization of coffee roasting process: Model development, prediction, optimization and application to upgrading of Robusta coffee beans

Since coffee bean roasting is a complicated process involving transient transport processes along with complex chemical reactions, modeling and optimizing such process is a challenge. Here, machine learning was first used to formulate models that allowed predictions of selected quality indicators of coffee beans undergoing hot air or superheated steam roasting at various conditions. Starling particle swarm optimization (SPSO) as well as other swarm intelligence and gradient-based algorithms were then used to determine conditions that would yield roasted beans with quality indicators similar to those of benchmarks. Test was also performed to determine if Robusta beans could be roasted at conditions depicted by SPSO to yield the beans with quality indicators similar to those of commercial blend of Arabica and Robusta beans. SPSO predicted values of quality indicators with average errors of lower than 9% and 13% when laboratory-scaled Robusta beans and commercial blend of beans were used as benchmarks.

Large-scale volumetric flow studies on transport of aerosol particles using a breathing human model with and without face protections

Exhalation of small aerosol particle droplets and their airborne transport, dispersion, and (local) accumulation in closed rooms have been identified as the main pathways for direct and indirect respiratory virus transmission from person to person, for example, for severe acute respiratory syndrome coronavirus-2 or measles. Therefore, understanding airborne transport mechanisms of aerosol particles inside closed populated rooms is an important key factor for assessing and optimizing various mitigation strategies. Unsteady flow features, which are typically evolving in such mixed convection flow scenarios, govern the respective particle transport properties. Experimental and numerical methods that enable capturing the related broad range of scales in such internal flows over many cubic meters in order to provide reliable data for the adaptation of proper mitigation measures (distances, masks, shields, air purifiers, ventilation systems, etc.) are required. In the present work, we show results of a large-scale, three-dimensional Lagrangian particle tracking (LPT) experiment, which has been performed in a 12-m3 generic test room capturing up to 3 × 106 long-lived and nearly neutrally buoyant helium-filled soap bubbles (HFSBs) with a mean diameter of dHFSB ∼370 μm as (almost) passive tracers. HFSBs are used as fluid mechanical replacements for small aerosol particles dP < 5 μm, which allow to resolve the Lagrangian transport properties and related unsteady flow field inside the whole room around a cyclically breathing thermal manikin with and without mouth-nose-masks and shields applied. Six high-resolution complementary metal-oxide semiconductor streaming cameras, a large array of powerful pulsed light emitting diodes, and the variable-time step Shake-The-Box LPT algorithm have been applied in this experimental study of internal flows in order to gain insight into the complex transient and turbulent aerosol particle transport and dispersion processes around a seated and breathing human model.

Large-scale 3D flow investigations around a cyclically breathing thermal manikin in a 12 m³ room using HFSB and STB

Exhalation of small aerosol droplets and their transport, dispersion and (local) accumulation in closed rooms have been identified as the main pathway for indirect or airborne respiratory virus transmission from person to person, e.g. for SARS-CoV 2 or measles (Morawska and Cao 2020). Understanding airborne transport mechanisms of viruses via small bio-aerosol particles inside closed populated rooms is an important key factor for optimizing various mitigation strategies (Morawska et al. 2020), which can play an important role for damping the infection dynamics of any future and the ongoing present pandemic scenario, which unfortunately, is still threatening due to the spreading of several SARS-CoV2 variants of concern, e.g. delta (Kupferschmidt and Wadman 2021). Therefore, a large-scale 3D Lagrangian Particle Tracking experiment using up to 3 million long lived and nearly neutrally buoyant helium-filled soap bubbles (HFSB) with a mean diameter of ~ 370 µm as passive tracers in a 12 m³ generic test room has been performed, which allows to fully resolve the Lagrangian transport properties and flow field inside the whole room around a cyclically breathing thermal manikin (Lange et al. 2012) with and without mouth-nose-masks and shields applied. Six high-resolution CMOS streaming cameras, a large array of powerful pulsed LEDs have been used and the Shake-The-Box (STB) (Schanz et al. 2016) Lagrangian particle tracking algorithm has been applied in this experimental study of internal flows in order to gain insight into the complex transient and turbulent aerosol particle transport and dispersion processes around seated breathing persons.

Efficient perturbation-tracking method for directly probing the spectral phonon properties from molecular dynamics simulations.

Existing methods for directly extracting the spectral phonon properties from molecular dynamics (MD) simulations, like the normal mode analysis (NMA) and spectral energy density analysis, all require a very long simulation time to produce reliable results with good convergence. So far, these methods are mainly applied in studies using small systems and with empirical potentials, as the heavy computational load has greatly hindered their further applications. Here we propose a perturbation-tracking (PT) method for directly probing the mode-wise phonon anharmonic frequencies and lifetimes. We show that results obtained from our method are in excellent agreement with those from the conventional NMA approach, using Si as the model material system. Comparing with the NMA approach, the PT method offers a greater accuracy and significant improvement of efficiency. It takes an average of two orders of magnitude and up to three orders of magnitude less simulation time to obtain the same lifetime result of a phonon mode with intermediate to high accuracy. Meanwhile, our method preserves all the dynamics of probed phonon mode from a particular state, which means it is capable of studying the transient thermal transport processes in a nonequilibrium system. Besides the exceptional efficiency, our method also comes with freedom to choose to probe only those modes of interest. This makes it ideal for use with large systems and in computationally demanding applications, such as ab initio MD simulations. Moreover, the PT method we propose here is very straightforward and easy to implement.

Electron transport within bulk cubic boron nitride: A Monte Carlo simulation analysis

Through the use of a semi-classical three-valley Monte Carlo electron transport simulation analysis, we analyze the nature of the steady-state and transient electron transport processes that occur within the zinc-blende phase of bulk boron nitride. For the purposes of our steady-state analysis, the dependence of the electron drift velocity on the applied electric field strength is examined. For our transient electron transport analysis, however, we study how an ensemble of electrons, initially in thermal equilibrium, i.e., zero-field, responds to the sudden application of a constant and uniform applied electric field. The results obtained, corresponding to the specific case of bulk zinc-blende boron nitride, are then contrasted with those corresponding to a number of other compound semiconductors of interest within the III–V nitride semiconductor genome.

Dynamic Properties of Particle Injections Inside Geosynchronous Orbit: A Multisatellite Case Study.

Four closely located satellites at and inside geosynchronous orbit (GEO) provided a great opportunity to study the dynamical evolution and spatial scale of premidnight energetic particle injections inside GEO during a moderate substorm on 23 December 2016. Just following the substorm onset, the four spacecraft, a LANL satellite at GEO, the two Van Allen Probes (also called “RBSP”) at ~5.8 R E, and a THEMIS satellite at ~5.3 R E, observed substorm‐related particle injections and local dipolarizations near the central meridian (~22 MLT) of a wedge‐like current system. The large‐scale evolution of the electron and ion (H, He, and O) injections was almost identical at the two RBSP spacecraft with ~0.5 R E apart. However, the initial short‐timescale particle injections exhibited a striking difference between RBSP‐A and ‐B: RBSP‐B observed an energy dispersionless injection which occurred concurrently with a transient, strong dipolarization front (DF) with a peak‐to‐peak amplitude of ~25 nT over ~25 s; RBSP‐A measured a dispersed/weaker injection with no corresponding DF. The spatiotemporally localized DF was accompanied by an impulsive, westward electric field (~20 mV m−1). The fast, impulsive E × B drift caused the radial transport of the electron and ion injection regions from GEO to ~5.8 R E. The penetrating DF fields significantly altered the rapid energy‐ and pitch angle‐dependent flux changes of the electrons and the H and He ions inside GEO. Such flux distributions could reflect the transient DF‐related particle acceleration and/or transport processes occurring inside GEO. In contrast, O ions were little affected by the DF fields.

Investigation of transient metal vapour transport processes in helium arc welding by imaging spectroscopy

In order to understand the transient transport mechanisms of metal vapour in an arc plasma, imaging spectroscopy was performed for helium arc plasmas burning on pure metals. The measured images were converted into the spectral intensity distributions at the central cross-section of the arc plasma by image processing and Abel inversion. The transient transport process of metal vapours inside the arc plasma are discussed in terms of the diffusion coefficients of metal vapour. The metal vapour generated from the weld pool surface was transported into the arc plasma and to the region near the tungsten electrode within a few seconds. As time passed, the metal vapour spectral intensities gradually increased near the weld pool surface, near the electrode and in the intermediate region. In contrast, the He I spectral intensity was weak and the He I distribution shrunk toward the centre of the arc. This is because the helium arc plasma was cooled by the strong radiation from metal vapour. For an arc burning on pure chromium, chromium ions reached to centre of the arc after about 40 s. In contrast, on pure iron, iron ions did not reach to the centre of the arc. This is because the helium arc plasma on pure chromium was cooled to a temperature at which the electric field and ordinary diffusion coefficients of chromium are largest, whereas the temperature of the plasma on pure iron remained high. Two factors contributed to this; the higher boiling point of iron, which reduces the iron vapour concentration, and the lower radiative emission coefficient of iron.

Least action principles for irreversible transport processes

Least action principles (i.e., variational principles) were firstly applied to describe reversible processes, such as geometrical optics and mechanics. Then, they were extended to handle irreversible processes on the basis of entropy production rate, and the principle of least dissipation of energy by Onsager is one of the representative theorems. Nevertheless, it should be noted that the least action principles based on entropy production rate cannot always derive the actual constitutive relation of an irreversible transport process. As an example, the least action principles based on entropy production rate, including the principle of least dissipation of energy, will not derive the Fourier’s heat conduction law that is the fundamental constitutive relation of thermal transport process. In order to resolve this problem, Guo et al. proposed a new quantity, entransy dissipation rate, which is defined as the production of heat flux and negative temperature gradient and proportional to the dissipation rate of the potential energy of thermomass. It has been proven that the action of heat conduction should be the entransy dissipation rate; moreover, a theoretical framework for heat transfer optimization has been built based on it. In the present work, we extend the entransy theory to deal with various irreversible transport processes, including mass and momentum transport, and analyze their least action principles and Lyapunov functions. It is found that the action for a specific irreversible transport process can be constructed using the Rayleigh dissipation function and the product of actual driving force and flux, which can be regarded as the dissipation of some type of energy. For instance, the action for the momentum transport process governed by Newton’s viscosity law can be constructed using the dissipation of mechanic energy, while the action for mass transport process described by Fick’s law is related to the dissipation of chemical potential that is the energy absorbed or released due to a change of the particle number of the given species. Lyapunov function reflects the irreversibility of a non-equilibrium transport process; we find that entropy production rate can serve as a Lyapunov function merely for an irreversible transport process governed by a constitutive relation in which the production of force and flux is equal to the entropy production rate, that is, the constitutive relation of entropy representation. It is noted that the constitutive relation of entropy representation cannot always coincide with the actual constitutive relation in practice. For example, heat conduction process is generally regarded as being governed by Fourier’s law; however, its constitutive relation of entropy representation can agree with the Fourier’s law only when the thermal conductivity is inversely proportional to the square of temperature. In fact, for an actual transport process, its Lyapunov function should be identical to its action that can derive the actual constitutive relation. It is the entransy dissipation rate that serves as both the action and Lyapunov function of thermal transport process. Lastly, we note that the combination of action and energy conservation equation cannot give the governing equation for a transient irreversible transport process; the quasi-least action principle can be obtained by introducing convolution integral.

Transient gas diffusivity evaluation and modeling for methane and helium in coal

Gas diffusion in coal in known to be a transient process. A conceptualized cylindrical pore network was proposed in this study to quantify the transient mass transfer in coal at various pressures. Experimental measurements were carried out to measure the time-dependent mass transfer through the volumetric method. Based on the geometrical model, a new analytical model was proposed to describe and quantify the dynamic contributions of bulk diffusion, Knudsen diffusion and surface diffusion to the total gas mass transfer. In the model, the nanoscale pore deformation and dynamics were implicitly considered and modeled through the pore volume compressibility and the matrix shrinkage and swelling variations. This allows to model the gas transient process with pore structure modification. The analytical model is being solved through a numerical approach. The numerical model also combines the interactions between gas adsorption and desorption by recalling the converse source terms in the governing equations. The transient gas transport process can be divided into two stages, namely, the fast bulk and Knudsen diffusion controlled free pressure equilibrium and the slow sorption-controlled mass transfer due to the gradient of free gas pressure (pf) and adsorption equivalent pressure (pa). The numerical model was calibrated and validated against with the laboratory measurements. Comparing the cases that helium and methane injections at similar pressure (~315.98 psi for helium and ~319.90 psi for methane), the equilibrium time for helium is ~ 4.5% shorter than that of methane does attributed to the relative slow sorption kinetics. Also, the pore radius with methane injection will never recover to the initial pore radius, a 3.3% irreversible strain on pore radius induced by sorption effects was observed. A 3.6% difference between the relative probability of the molecular-wall collisions for helium (~97.7%) and the relative probability of molecular-wall collisions between methane molecules (~ 94.1%) was believed to be induced by the sorption effects. These results can ultimately provide the approach and data for analyzing the dynamic sorbing gas transport behaviors in sorptive coal.

Laboratory and Numerical Investigation of Saltwater Intrusion Processes in a Circular Island Aquifer

AbstractA novel radial flow experimental system to study saltwater intrusion processes in an island aquifer is presented. The study investigated steady state and transient scenarios involving advancing and receding saltwater wedges in a circular island. The experimental results were simulated using the density‐coupled version of the MODFLOW‐USG code. The experimental data along with the model simulation results are employed to develop a new radial benchmark problem for testing density‐coupled models used for simulating saltwater intrusion processes. The experimental data for transient changes in toe position and freshwater storage level indicated an asymmetric pattern where the intrusion time scale is greater than the recession time scale. Numerical experiments were completed to further investigate this asymmetric effect and to intercompare the associated transient transport processes in circular and linear strip islands. We also analyzed the sensitivity of island geometry in controlling the freshwater storage levels under different recharge conditions. Modeling results show that for similar‐sized systems, circular islands are more efficient in storing freshwater than linear strip islands.

A pore network model of porous electrodes in electrochemical devices

• A PNM is computationally efficient for analysing flow and transport processes in electrodes. • The concentration profile of electrolyte is non-uniform across the 3D electrode. • Pore size distribution and connectivity influence the intrusion of electrolyte in the electrode. • Current density across the electrode is non-uniform and related to the concentration profiles. A computationally efficient pore network model (PNM) has been developed to incorporate the transport and electrochemical phenomena occurring within porous electrodes. This PNM is validated on a synthetic cubic structure and subsequently run on a network obtained from X-ray computed tomography (X-CT) images of a sample of commercial porous carbon paper commonly used in electrochemical devices. The carbon paper's physical characteristics (pore-size distribution, permeability, porosity and electroactive surface area) are discussed. The concentration distribution of active species is examined considering solely the transient convective and diffusive transport processes initially, and subsequently is compared to the concentration of active species when migration and reactive transport factors are included. The results show non-uniformity in the concentration and pressure distributions in the electrode when considering the pure convective/diffusive transport processes. The migration and reactive processes are subsequently considered and are shown to be influenced by the rate in which the convective/diffusive flow permeates the electrode. A uniform steady decline in volume-averaged state of charge is shown, followed by a pore-scale non-uniform current density and state of charge distribution upon discharge. These results were obtained on a standard single core workstation highlighting the benefits of using a computationally inexpensive model.

Discrete unified gas kinetic scheme for multiscale heat transfer with arbitrary temperature difference

In this paper, a finite-volume discrete unified gas kinetic scheme (DUGKS) based on the non-gray phonon transport model is developed for multiscale heat transfer problem with arbitrary temperature difference. Under large temperature difference, the phonon Boltzmann transport equation (BTE) is essentially multiscale, not only in the frequency space, but also in the spatial space. In order to realize the efficient coupling of the multiscale phonon transport, the phonon scattering and advection are coupled together in the present scheme on the reconstruction of the distribution function at the cell interface. The Newtonian method is adopted to solve the nonlinear scattering term for the update of the temperature at both the cell center and interface. In addition, the energy at the cell center is updated by a macroscopic equation instead of taking the moment of the distribution function, which enhances the numerical conservation. Numerical results prove that the present scheme can describe the multiscale heat transfer phenomena accurately with arbitrary temperature difference in a wide range. In the diffusive regime, even if the time step is larger than the relaxation time, the present scheme can capture the transient thermal transport process accurately. Compared to that under small temperature differences, as the temperature difference increases, the variation of the temperature distribution behaves quite differently and the average temperature in the domain increases in the ballistic regime but decreases in the diffusive regime.

(Invited) Understanding Redox Shuttle Photocatalysis in Z-Scheme Solar Water Splitting Reactors

Particle suspension reactors for solar water splitting are capable of generating hydrogen at a cost that is competitive with hydrogen produced from steam methane reforming.1-3 Our team has validated a reactor design that resembles Nature’s Z-scheme where two stacked and connected photocatalyst particle suspension reactor beds together drive overall solar water splitting.3 Electron (and proton) management between the beds occurs by transport of a redox shuttle through a nanoporous separator. Efficient designs require that the redox shuttle is selectively oxidized and reduced at the particles that drive H2 evolution and O2 evolution, respectively. By device physics numerical simulations we showed that even for highly efficient reactor designs (10% STH efficiency) redox shuttle transport between the beds can be sustained with only passive diffusion.3 In my presentation I will report on our team’s recent progress on this design. Using finite-element numerical analyses we modelled and simulated the transient mass transport processes, light absorption, electrochemical kinetics, gas crossover, and thermal transport in the proposed reactor. Experimentally, we synthesized, characterized, and evaluated the photo(electro)chemical performance of the most promising photocatalyst nanocrystallites (BiVO4, WO3, and Rh-doped SrTiO3) as mesoporous thin films and as particles in model reactors, and in the presence of several different redox shuttles and at various pH values. For H2-evolving Rh-doped SrTiO3, we demonstrated that in the presence of Fe(II) the limiting rate of Fe(III) reduction decreases and the rate of H2 evolution increases; however, these desired processes occurred along with undesired Fe(III) reduction and undesired H2 oxidation. Introduction of Ru cocatalysts enhanced performance by increasing the rate of H2 evolution and to a lesser extent undesired Fe(III) reduction. For O2-evolving WO3, we showed that O2 does not interfere with collection of electrons and that selectivity toward Fe(III) reduction is possible at moderate concentrations of Fe(III). Overall, results from several studies using a series of redox shuttles and photocatalyst particles will be presented. Collectively, our efforts represent strides toward achieving a high-level of techno-economic viability in solar water splitting reactors. Acknowledgments: This work was supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Incubator Program under Award No. DE-EE0006963 and Lawrence Berkeley National Laboratory under Contract No. DE-AC02-05CH11231.

Recent progress in the concurrent atomistic-continuum method and its application in phonon transport

Abstract