Medium Energy Storage Research Articles

3D small-gain formula allowing strong focusing and harmonic lasing for a ring-based x-ray free electron laser oscillator

We present a detailed derivation of a formula for the small-gain calculation for an x-ray free electron laser oscillator (XFELO) based on a medium-energy (3–4 GeV) storage ring. We found harmonic lasing and strong focusing are essential for this beam energy range. Taking the small-signal low-gain formula developed by Kim and his colleagues, we modified it in such a way that the gain can be calculated without the “no focusing approximation,” and a strong focusing can be applied, as well as harmonic lasing. In this formula, the gain is represented as a product of two factors with one of them depending only on the harmonic number, undulator period, and gap. Using this factor, we show that it is favorable to use harmonic lasing to achieve hard x-ray FEL working in the small-signal low-gain regime with the medium-energy electron beam. Our formula also allows FEL optimization by varying the vertical gradient of the undulator, the vertical dispersion, and the horizontal and vertical focusing, independently. As an example, we applied this formula to study the feasibility of an XFELO option for the National Synchrotron Light Source II (NSLS-II) upgrade. Since a quite high peak current is required for the FEL, collective effects of beam dynamics in medium-energy synchrotrons significantly affect the electron beam parameters. We carried out a multiparameter optimization taking collective effects into account. Note, even though our example is for a ring-based XFELO at 3 to 4 GeV, the formula and, in particular, the approach developed here may be applied to other types of FELs. Published by the American Physical Society 2024

Development and comparison of multi-walled carbon nanotubes and graphite nanoflakes dispersed solar salt: Structural formation and thermophysical properties

A molten salt energy storage has proven to be a promising candidate for medium and high-temperature thermal energy storage (TES) applications. Improving the thermophysical properties, such as specific heat capacity (Cp) and thermal conductivity (k) is crucial to deciding the charging/discharging characteristics of the energy storage system. In this study, 1 wt% of two variant carbon allotropes, such as multi-walled carbon nanotubes (MWCNTs) and graphite nanoflakes (GNFs), were dispersed in solar salt (i.e., MWCNTs nanosalt and GNFs nanosalt) using wet preparation method. Thermophysical properties of the prepared nanosalts were evaluated using different analytical techniques and compared with pristine solar salt. The specific heat capacity was measured from 50 °C to 350 °C, and the maximum specific heat capacity enhancement in 1 wt% of MWCNTs dispersed solar salt was 18 %. The enhancement in 1 wt% of GNFs dispersed solar salt was 12.5 % at 200 °C (in solid phase). Similarly, in the liquid phase (at 350 °C), MWCNTs dispersed solar salt improves Cp by 6.2 %, while GNFs dispersed solar salt exhibits negligible improvement in Cp. Further, a modified transient plane source (MTPS) technique was used to measure the thermal conductivity of both carbon nanosalts in the solid phase (30–100 °C). The maximum thermal conductivity enhancement observed for MWCNTs dispersed solar salt and GNFs dispersed solar salt was 5.8 % and 11.7 %. Finally, thermogravimetric analysis was conducted to evaluate the thermal stability of the MWCNTs nanosalt and GNFs nanosalt at a maximum temperature of 700 °C, and found that the MWCNTs nanosalt show better thermal stability than the GNFs nanosalt.

Adiabatic Compressed Air Energy Storage system performance with application-oriented designed axial-flow compressor

Medium and long-duration energy storage systems are expected to play a critical role in the transition towards electrical grids powered by renewable energy sources. ACAES is a promising solution, capable of handling power and energy ratings over hundreds of MW and MWh, respectively. One challenge with ACAES is achieving the required highly efficient operation in the compressor over the range of conditions encountered in the system as the pressure in the air store changes. In this paper, an application-oriented axial-flow compressor is designed, aiming towards efficient operation throughout the operation range, whilst also associating the performance prediction to a practical compressor geometry. A two-step design methodology based on inviscid, axisymmetric flow conditions has been implemented, leading to the flowtrack, blade-row geometries and the compressor performance map. The compressor model is integrated into an ACAES model, including two compression spools, two expansion stages with preheat, a constant volume high pressure storage operating between 5.5 and 7.7 MPa and two separate Thermal Energy Storage units. While the existing ACAES literature either ignores the transient off-design operation or uses generic numerical correlations (which are not associated to a particular geometry), the key novelty of this paper is the application of a detailed design method for turbomachinery to ACAES. The results indicate that the designed compressor requires 33 stages over the two spools, and is able to operate efficiently over the storage pressure range, showing that if the application-oriented design procedure is applied to the compressor, it does not stop ACAES reaching 70% round-trip efficiency, outputting 35MW for approximately 15 h. Importantly, the specific ACAES requirement of conserving heat at higher temperatures has been fulfilled by decreasing the number of intercoolers. Finally, it is recommended that a similar level of scrutiny is applied to the other components (i.e. expanders, heat exchangers and TES units), keeping in mind the unique set of operational requirements of ACAES. This work is an important step towards removing the common misconception that off-the-shelf components can be easily be used in typical ACAES designs.

Investigation of water adsorption characteristics of MgCl2 salt hydrates for thermal energy storage: Roles of hydrogen bond networks and hydration degrees

Thermochemical energy storage holds great promise in solar energy applications, and MgCl2 hydrate salt is considered a promising material for medium and low-temperature thermochemical energy storage. Understanding the adsorption behavior of water molecules in MgCl2 hydrate salts and uncovering the underlying mechanisms are crucial for designing efficient thermochemical energy storage systems. This paper investigated the water adsorption characteristics of MgCl2 hydrate salt utilizing the Grand Canonical Monte Carlo (GCMC) method and captured the adsorption isotherm and concentration distributions of water molecules in MgCl2 hydrate salts pores at various temperatures. The results demonstrate that at low temperatures, particularly when the pore size is small (D < 0.8 nm), the interconnected hydrogen bond network plays a crucial role in enhancing the water adsorption capacity of pure MgCl2. As the temperature increases, the influence of hydrogen bond networks diminishes, and their impact on the water adsorption capacity becomes negligible. Moreover, the hydration degrees of MgCl2 salt significantly influence their water adsorption capacity. MgCl2·6H2O exhibits the best water-uptake performance, followed by MgCl2·4H2O, while MgCl2·2H2O displays the worst water-uptake performance. These findings shed light on the adsorption behavior of water molecules in MgCl2 hydrate salt and offer valuable insights into the design and optimization of efficient thermochemical energy storage systems. Such knowledge is essential for advancing the utilization of thermochemical energy storage in solar energy applications.

Long-Life Lead-Carbon Batteries for Stationary Energy Storage Applications.

Owing to the mature technology, natural abundance of raw materials, high recycling efficiency, cost-effectiveness, and high safety of lead-acid batteries (LABs) have received much more attention from large to medium energy storage systems for many years. Lead carbon batteries (LCBs) offer exceptional performance at the high-rate partial state of charge (HRPSoC) and higher charge acceptance than LAB, making them promising for hybrid electric vehicles and stationary energy storage applications. Despite that, adding carbon to the negative active electrode considerably enhances the electrochemical performance. However, carbon brings some adverse effects, such as the severe hydrogen evolution reaction (HER) in the NAM due to the low overpotential of carbon material, promoting severe water loss in LCBs. From a practical application point of view, the irreversible sulfation of the negative active material (NAM) and extreme shedding and softening of the positive active material (PAM) are the main obstacles for next-generation LCBs. Recently, a lead-carbon composite additive delayed the parasitic hydrogen evolution and eliminated the sulfation problem, ensuring a long life of LCBs for practical aspects. This comprehensive review outlines a brief developmental historical background of LAB, its shifting towards LCB, the failure mode of LAB, and possible potential solutions to tackle the failure problems. The detailed LCB's development towards long life was discussed in light of the reported literature to guide the researcher to date progress. More emphasis was directed toward the new applications of LCBs for stationary energy storage applications. Finally, state-of-the-art progress and further research gaps were pointed out for future work in this exciting era.

Three-pole wigglers at NSLS-II

The National Synchrotron Light Source-II (NSLS-II) at Brookhaven National Lab (BNL) is a third-generation medium-energy storage ring with 3 GeV and sub-nm-rad horizontal emittance, equipped with a top-off capability of 500 mA. It is designed to deliver an extremely intense photon beam across a wide spectral range, from the far-infrared to the very hard X-ray region. This outstanding performance is achieved through a combination of bending magnets (BM), three-pole wigglers (3 PW), and advanced insertion device (ID) sources. Six three-pole wigglers have been constructed at the NSLS-II IDs Lab. These devices are used to produce broadband radiation with lower angular power density and to monitor the electron beam emittance and energy spread. This paper describes the R&D activities focused on developing the required magnetic and mechanical designs, along with magnetic field optimization and the final magnetic measurement results. It also covers the spectral brightness, flux, power density, spatial and angular output properties of the 3 PWs, including their commissioning results and their effects on the performance of the NSLS-II storage ring.

Thermal performance enhancement of high durability alloy microencapsulated phase change material (MEPCM)/ceramic composites based on industrial graphene

Metallic phase change material (PCM)/ceramic composites have emerged as promising candidates for medium and high-temperature thermal energy storage (TES) due to their excellent performance. However, challenges associated with leakage and oxidation in metals, along with the limited thermal conductivity of ceramic matrices, restrict further performance enhancement. Herein, an alloy microencapsulated phase change material (MEPCM) is prepared as a PCM using the “double-layer coating, sacrificial inner layer” method, followed by compounding it with a ceramic matrix and industrial graphene (IG). Based on this, a novel IG/SnBi58 MEPCM/ceramic composite is prepared and its performances are investigated. The results show that the composite with 12 wt% IG exhibits optimum performance: a thermal conductivity of 5.75 W/(m·K), 86.1% higher than without IG, melting enthalpy of 73.71 J/cm3, and compressive strength of 49.38 MPa. Furthermore, the microencapsulation of alloys fundamentally solves the PCM leakage issue within the composite and enables its good component compatibility. The heat storage density reaches 399.1 MJ/m3, over 2.3 times that of traditional quartz sensible heat storage materials. Owing to the thermally expanded cavities created by “double-layer coating, sacrificial inner layer” method, the prepared MEPCM demonstrates high thermal reliability, so the composite maintains stable thermal properties and chemical structure after 500 thermal cycles. Finally, the comparative analysis reveals that the composite combines higher thermal conductivity, compressive strength, heat storage density, and thermal cycle stability than other composite heat storage materials. This work provides a new strategy to meet the demand for high-performance heat storage materials in medium-temperature TES applications.

Heat discharge performance of metal hydride thermal battery under different heat transfer conditions: Experimental findings

Thermal batteries utilizing metal hydride pairs are gaining tremendous research appeal for their applications in thermochemical energy storage. The pair consisting of a high-temperature metal hydride (HTMH: Mg-based hydride) and a low-temperature metal hydride (LTMH: AB2 type hydride) is attractive due to its relatively high energy storage density and medium energy storage efficiency. The energy storage efficiency can be significantly increased by improving the heat discharging performance of the thermal battery. In this study, we experimentally explore the heat releasing performance of an MgH2/(TiZr)(MnFeCr)2-based thermal battery. More specifically, the effects of LTMH bed heat transfer conditions on the heat discharging performance were briefly discussed. These heat transfer conditions include natural convection, forced convection, and resistive heating. The results showed that when operating the LTMH bed under active heat transfer conditions (forced convection or resistive heating), the thermochemical energy storage density varies between 1500 and 1820 kJ/kg-Mg with relatively high-temperature lift (heat upgrade) between 47 and 55 °C. On the other side, the thermal battery discharges heat at a relatively high specific power, ca. 100–225 W/kg-Mg, which can be a benefit for heat-to-work conversion applications using organic or steam Rankine cycles.

Longitudinal stacking of ion beams with pulsed electron beam cooling

Longitudinal ion beam stacking with the combination of barrier rf system and beam cooling has been demonstrated in several experiments. Based on the bunching effect found in the pulsed electron beam cooling experiment at HIRFL-CSRm, we propose a new beam stacking scheme using only pulsed electron beam, in which the barrier voltage and cooling effect can be realized simultaneously. In this paper, we introduce this longitudinal stacking scheme along with the theory of beam dynamics and present a simple analytical model. The simulation demonstrates that this approach could be a useful beam stacking technique without the need for barrier bucket hardware. Moreover, the optimization and limitation of this stacking scheme are discussed, and the effect of the electron beam distribution on the barrier voltage is studied. We expect this stacking method can be applied to accumulate RIBs in low and medium energy storage rings for high-precision experiments, such as the SRing of the HIAF project.

Assessment of an intermediate working medium and cold energy storage (IWM-CES) system for LNG cold energy utilization under real regasification case

Owing to the fluctuating liquified natural gas (LNG) regasification rates in receiving terminals, LNG cold energy utilization systems face low recovery rate issue. In this work, an intermediate working medium and cold energy storage (IWM-CES) system is proposed to improve the operational flexibility and recovery rate for LNG cold energy utilization under actual regasification case. The proposed system comprises two modules, namely, cold energy recovery module and cold energy storage module. The cold energy recovery module recovers deep and shallow LNG cold energies using intermediate working mediums. The cold energy storage module acts as a buffer to promote stable cooling output. Key results revealed the maximum stable cooling output of conventional system (without cold energy storage) is constrained by the minimum regasification rate, which causes a relatively low recovery rate. On the contrary, the proposed system realizes a larger stable cooling output and enhances the cold recovery rate from 33.2% to 80.3%. Economically, the proposed system requires the injection of additional capital cost but can achieve better lifecycle profit. Environmental benefits are conducted and ultimately evaluated from users' perspective. Compared to the conventional system, the proposed system achieves the improvement of daily CO2 emissions reduction spanning 75.2 tons/day to 453.8 tons/day.

Comparative review of different influence factors on molten salt corrosion characteristics for thermal energy storage

As high-temperature heat transfer medium and thermal energy storage material, molten salts are widely applied in concentrating solar power (CSP) plants and molten salt reactors (MSR). The molten salt corrosion is a crucial influence factor for the system safety operation. Different influence factors such as temperature, impurity, alloy composition, gas atmosphere, nanofluids and flow state on molten salt corrosion are comparatively summarized in this work. Meanwhile, the corrosion mechanisms of nitrate, carbonate, chloride and fluoride salts are emphatically reviewed. Furthermore, the anti-corrosion strategies of metallic materials to molten salt are presented. Finally, the future research directions of molten salt corrosion characteristics are proposed. This work could provide practical guidance for the application selection of metal materials in the CSP and MSR system.

Thermal Storage of Nitrate Salts as Phase Change Materials (PCMs).

This study presents the energy storage potential of nitrate salts for specific applications in energy systems that use renewable resources. For this, the thermal, chemical, and morphological characterization of 11 samples of nitrate salts as phase change materials (PCM) was conducted. Specifically, sodium nitrate (NaNO3), sodium nitrite (NaNO2), and potassium nitrate (KNO3) were considered as base materials; and various binary and ternary mixtures were evaluated. For the evaluation of the materials, differential Fourier transform infrared spectroscopy (FTIR), scanning calorimetry (DSC), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) to identify the temperature and enthalpy of phase change, thermal stability, microstructure, and the identification of functional groups were applied. Among the relevant results, sodium nitrite presented the highest phase change enthalpy of 220.7 J/g, and the mixture of 50% NaNO3 and 50% NaNO2 presented an enthalpy of 185.6 J/g with a phase change start and end temperature of 228.4 and 238.6 °C, respectively. This result indicates that sodium nitrite mixtures allow the thermal storage capacity of PCMs to increase. In conclusion, these materials are suitable for medium and high-temperature thermal energy storage systems due to their thermal and chemical stability, and high thermal storage capacity.

Performance Evaluation of Coupled Thermal Enhancement through Novel Wire-Wound Fins Design and Graphene Nano-Platelets in Shell-and-Tube Latent Heat Storage System

Technological development in latent heat storage (LHS) systems is essential for energy security and energy management for both renewable and non-renewable sources. In this article, numerical analyses on a shell-and-tube-based LHS system with coupled thermal enhancement through extended fins and nano-additives are conducted to propose optimal combinations for guaranteed higher discharging rate, enthalpy capacity and thermal distribution. Transient numerical simulations of fourteen scenarios with varied combinations are investigated in three-dimensional computational models. The shell-and-tube includes paraffin as phase change material (PCM), longitudinal, radial and wire-wound fins and graphene nano-platelets (GNP) as extended fins and nano-additives, respectively. The extended fins have demonstrated better effectiveness than nano-additives. For instance, the discharging durations for paraffin with longitudinal, radial and wire-wound fins are shortened by 88.76%, 95.13% and 96.44% as compared to 39.33% for paraffin with 2.5% GNP. The combined strengths of extended fins and nano-additives have indicated further enhancement in neutralising the insulative resistance and stratification of paraffin. However, the increase in volume fraction from 1% to 3% and 5% is rather detrimental to the total enthalpy capacity. Hence, the novel designed wire-wound fins with both base paraffin and paraffin with 1% GNP are proposed as optimal candidates owing to their significantly higher heat transfer potentials. The proposed novel designed configuration can retrieve 11.15 MJ of thermal enthalpy in 1.08 h as compared to 44.5 h for paraffin in a conventional shell-and-tube without fins. In addition, the proposed novel designed LHS systems have prolonged service life with zero maintenance and flexible scalability to meet both medium and large-scale energy storage demands.

Techno-economic analysis on the design of sensible and latent heat thermal energy storage systems for concentrated solar power plants

One feasible solution to reduce the cost of concentrated solar power is to employ a higher temperature and efficiency supercritical carbon dioxide Brayton power cycle. An alternative heat transfer medium and thermal energy storage (TES) system therefore needs to be explored. This work considered a variety of phase change materials (PCMs) and graphite as the storage media in four indirect shell-and-tube storage configurations, including a 3-PCM and 5-PCM cascade storage, a PCM-graphite-PCM hybrid storage and a single graphite storage. The sizing and design of the TES systems were performed by using a dynamic cycling methodology based on a transient 2D numerical model. The cost of those designs were then determined by using an economic model developed inhouse. This work also investigates the impact of some geometric parameters and cost assumptions on the techno-economic performance of the TES system. The analysis suggests a scenario exists whereby a low thermal efficiency storage system which therefore utilises less tube or storage material could be more cost-effective. Overall, the cost of hybrid TES is the lowest among all studied systems, at $26.96/kWht and $21.49/kWht for charging temperature of 720 °C and 750 °C, respectively, followed by the 5-PCM storage at $28.06/kWht and $21.82/kWht, respectively.

A Literature Review on Novel Nitrate Molten Salt for Heat Storage

Reducing the melting point, in creasing the thermal stability limit, and enhancing the specific heat capacity of molten salt are the research hotspots in the field of medium and high temperature energy storage in recent years. From the perspectives of the melting point, thermal stability limit, and specific heat capacity of nitrates, we summarize the melting point, thermal stability limit, and specific heat capacity enhancement of molten salts with different compositions and ratios. The melting points of molten salt with different compositions and ratios are compared. Furthermore, the enhancing effect of various nanomaterials on molten salt is elucidated. The application of nitrate molten salt is also summarized to provide a reference for the research and application of novel molten salts. Keywords: Nitrate Molten Salt; Melting Point; Thermal Stability Limit; Specific Heat Capacity; Application

Experimental and numerical investigation on enhancing heat transfer performance of a phase change thermal storage tank

There are extensive research and application prospects in the fields of solar energy utilization and waste heat recovery of low and medium temperature latent thermal energy storage (TES), but the low thermal conductivity of phase change material (PCM) seriously weakens the thermal response rate and the rate of heat storage and release of TES. In order to solve this problem, this paper adopts a multi-scale heat transfer enhancement technique. The following parameters by grouping experiments are investigated: expanded graphite (EG) particle content, the diameter and placement position of copper rods. Then the experimental results are analyzed by the statistical response surface method (RSM). At the same time, a 3D model of the phase change heat storage of a single tank is established considering the natural convection of liquid PCM. Meanwhile, the influence of the placement of copper fin on the charging process was analyzed numerically. RSM analysis and simulation results show that copper rod diameter and EG particle content are two important parameters affecting the melting of PCM. Meanwhile, it is predicted that in the range of D Cu = 4-6mm, if the EG particle content is 2 wt.%, it has a shorter melting time and higher economic performance. The placement of the copper rod has no obvious effect on the charging process, but the placement of the annular copper fin has an obvious effect on the melting process of PCM. This study will provide guidance for the heat transfer optimization design of a single tank TES.

Hydrogen/Vanadium Hybrid Redox Flow Battery with enhanced electrolyte concentration

A high energy density Hydrogen/Vanadium (6 ​M HCl) system is demonstrated with increased vanadium concentration (2.5 ​M vs. 1 M), and standard cell potential (1.167 vs. 1.000 ​V) and high theoretical storage capacity (65 ​W h L−1) compared to previous vanadium systems. The system is enabled through the development and use of HER/HOR catalysts with improved chemical stability towards the halogen-containing electrolyte within which the usual catalyst (Pt/C) is shown to quickly degrade during potential hold experiments. The implementation of an Ir/C catalyst at the negative side enables a system with high achievable energy density of 45 ​W ​h ​L−1 at 75 ​mA ​cm−2 associated with 67% electrolyte utilization. Based on such a promising performance, the system here presented could be a suitable solution for medium and large-scale energy storage with lower cost and volume footprint than existing batteries, particularly all-vanadium RFBs.

Numerical Design Optimization of Short-Period HTS Staggered Array Undulators

Short period undulators are an essential component of a compact free electron laser (FEL) and medium energy storage rings for the production of hard X-rays. The use of ReBCO high temperature superconductors (HTS) in a staggered array undulator geometry is expected to yield a magnetic undulator flux density above 2 T for a 10 mm period and a 4 mm magnetic gap, thus substantially increasing the lasing performance of existing FEL and storage ring facilities. The optimization of the design for a staggered array undulator with FEM simulations and a critical analysis of its working principles are discussed in this paper. Specifically, the maximization of the undulator field and the minimization of the required HTS material are the two main objectives of this study, which is performed with realistic material parameters of commercially available bulks materials. Finally, the outcome of the applied optimization technique is discussed and the intrinsic limits of this undulator geometry are highlighted..

Visualising and Characterising Zinc Ion Transport for Zinc Ion Batteries By Fluorescence Microscopy

Zinc has been regarded as a promising anode material for aqueous batteries in view of its advantages including high specific capacity, abundance and intrinsic safety. Aqueous zinc ion batteries have attracted growing attention as a potential alternative to lithium ion batteries, especially for medium and large-scale energy storage. Aqueous zinc ion batteries consist of a zinc anode and a zinc intercalating cathode in a zinc-salt-containing electrolyte and use zinc ions as charge carriers. The electrolyte transporting zinc ions between the anode and the cathode plays an essential role in determining battery performance and life. Visualising ion transport in the electrolytes of zinc ion batteries can give insights into electrolyte dynamics as well as battery processes. Yet, this is limited by the spatial-temporal resolution of the existing techniques. Moreover, most of the existing in-situ visualisation techniques are very expensive and not easily accessible. Fluorescence microscopy provides a powerful tool for probing tiny structures and tracking species in real time. It relies on fluorescence which occurs when molecules absorb light with a certain wavelength followed by the re-emission of light with a longer wavelength. These excitation and emission wavelengths are usually unique fingerprints of certain substances. Fluorescent sensors, with high sensitivity of fluorescence assays, have been widely exploited as a useful tool for species detection and mechanistic studies in biology and chemistry. In this study, a novel microfluidics-based fluorescence microscopy platform is developed for visualising and characterising zinc ion transport in the electrolytes of zinc ion batteries. The platform is calibrated by comparing the measurement results with the literature data. Key transport properties of zinc ions are quantified under different electrolyte conditions. The developed platform is demonstrated to be a simple and versatile tool for electrolyte characterisation, which provides key information guiding future electrolyte development and battery performance improvement.

(Industrial Electrochemistry and Electrochemical Engineering Division Student Achievement Award Address) Fundamental Understanding on Cell Performance Enhancement of Flow Batteries with Serpentine Flow Field Structures

Redox flow batteries are being considered for medium and large-scale stationary energy storage applications. In addition to storing energy by the intermittent resources, such as tide, wind and solar energy, the electrochemical flow battery systems could impact electricity generation and grid effectiveness by supporting energy storage for peaking shaving, load leveling and emergency back up [1,2]. In recent years, flow field designs, such as a serpentine flow field, evolved from proton exchange membrane (PEM) fuel cell have been experimentally demonstrated to deliver improved cell performance and boost cell capacity [3] as compared to flow batteries without flow field designs. However, the mechanisms causing enhanced higher limiting current density with serpentine flow field designs have not been fully understood. In this presentation, I will introduce a concept of maximum current density associated with the stoichiometric availability of the pressure-driven electrolyte flow penetrating into the porous carbon electrode from the from the serpentine flow channel [4-6]. Our three-dimensional model demonstrates that this maximum current density concept can explain the observed limiting current density achieved in a flow battery with a serpentine flow field structure [7]. We hope this fundamental understanding can contribute to a better elucidation of the mechanism on limiting the electrochemical performance and further optimization of flow batteries. Acknowledgement This work was partially supported by the “all-iron flow battery” project with a grant number of DE-AR0000352 funded by the program of the Advanced Research Projects Agency-Energy (ARPA-E) of the Department of Energy (DOE) of the United States. I (XK) would like to acknowledge the late Professor Joseph M. Prahl, who was my advisor and inspirational mentor.