Vanadium Redox Flow Battery System Research Articles

Economic Operational Planning of Grid-connected Battery Energy Storage

In this paper, we develop a framework for valuing a battery energy storage system as it participates in the wholesale electricity market. We utilize an economic optimization routine with an internal battery energy storage model to plan when a battery should offer its power into the marketplace to maximize its benefit to the electric grid-and maximize its value. We examine the potential value of a vanadium redox flow battery (VRFB) used for frequency regulation service in the Texas organized electricity market from 2007-2009. Our results show that the price point where a VRFB system used for frequency regulation in Texas would be a good investment is approximately $1500/kW. We show that the expected annual revenue from frequency regulation is highly dependent on variations in wholesale prices, and that longer-duration storage could provide increased revenue from frequency regulation up to approximately 3 hours of storage capacity.

Species transport mechanisms governing capacity loss in vanadium flow batteries: Comparing Nafion® and sulfonated Radel membranes

In this study, a 2-D, transient vanadium redox flow battery (VRFB) model was used to investigate and compare the ion transport mechanisms responsible for vanadium crossover in Nafion® 117 and sulfonated Radel (s-Radel) membranes. Specifically, the model was used to distinguish the relative contribution of diffusion, migration, osmotic and electro-osmotic convection to the net vanadium crossover in Nafion® and s-Radel. Model simulations indicate that diffusion is the dominant mode of vanadium transport in Nafion®, whereas convection dominates the vanadium transport through s-Radel due to the lower vanadium permeability, and thus diffusivity of s-Radel. Among the convective transport modes, electro-osmotic convection (i.e., electro-osmotic drag) is found to govern the species crossover in s-Radel due to its higher fixed acid concentration and corresponding free ions in the membrane. Simulations also show that vanadium crossover in s-Radel changes direction during charge and discharge due to the change in the direction of electro-osmotic convection. This reversal in the direction of crossover during charge and discharge is found to result in significantly lower “net” crossover for s-Radel when compared to Nafion®. Comparison of these two membranes also provides guidance for minimizing crossover in VRFB systems and underscores the importance of measuring the hydraulic and the electro-kinetic permeability of a membrane in addition to vanadium diffusion characteristics, when evaluating new membranes for VRFB applications.

Kinetic Study of a Carbon Fiber Microelectrode in a Vanadium Redox Flow Battery System

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SPPEK/TPA composite membrane as a separator of vanadium redox flow battery

To improve the performance of a low cost sulfonated poly(phthalazinone ether ketone) (SPPEK) membrane in vanadium redox flow battery (VRFB), composite membranes composed of SPPEK and tungstophosphoric acid (TPA) with 8–25wt% concentration were prepared by solution casting. The results of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) of the SPPEK/TPA composite membrane revealed that TPA had excellent compatibility with SPPEK in the bulk of the membrane. The effect of the TPA concentration on the primary properties, permeability of vanadium ions and ionic selectivity, and chemical stability of the composite membrane was investigated. The single cell tests showed that the VRFB employing the SPPEK–TPA-17 membrane with 17wt% TPA exhibited a higher coulombic efficiency (98.75% vs. 92.81%) and energy efficiency (74.58% vs. 73.83%) than those of the Nafion system. Cycling and chemical stability tests indicated that the SPPEK/TPA composite membrane had high stability in the VRFB system.

1 kW/1 kWh advanced vanadium redox flow battery utilizing mixed acid electrolytes

This paper reports on the recent demonstration of an advanced vanadium redox flow battery (VRFB) using a newly developed mixed acid (sulfuric and hydrochloric acid) supporting electrolyte at a kW scale. The developed prototype VRFB system is capable of delivering more than 1.1 kW in the operation range of 15–85% state of charge (SOC) at 80 mA cm−2 with an energy efficiency of 82% and energy content of 1.4 kWh. The system operated stably without any precipitation at electrolyte temperatures >45 °C. At similar electrolyte temperatures, tests with a conventional sulfuric acid electrolyte suffered from precipitation after 80 cycles. By operating stably at elevated temperatures (>40 °C), the mixed acid system enables significant advantages over the conventional sulfate system, namely; 1) high stack energy efficiency due to better kinetics and lower electrolyte resistance, 2) lower viscosity resulting in reduced pumping losses, 3) lower capital cost by elimination of heat exchanger, 4) higher system efficiency and 5) simplified system design and operation. Demonstration of the prototype stack with the mixed acid electrolyte has been shown to lower the cost of conventional VRFB systems for large-scale energy storage applications.

Stability of Vanadium Electrolytes in the Vanadium Redox Flow Battery

ABSTRACTThe stability of the negative electrode electrolyte affects the efficiency and capacity of energy storage in the vanadium redox flow battery (VRFB) system. To explore the stability of vanadium electrolytes, the study prepared five types of V(II) electrolytes that were exposed to air in a fixed open area and monitored the charge state of vanadium ions over time by UV/Visible spectrophotometer. This study succeeded in preparing pure V(II) electrolytes. Five characteristics are found in the UV/Visible spectra, respectively, during the oxidation process from V(II) electrolytes to V(III) electrolytes and V(III) electrolytes to V(IV) electrolytes. The experimental results show that the oxidation rate of a solution of 1 M V(II) electrolytes to V(III) electrolytes and 1 M V(III) electrolytes to V(IV) electrolytes under an atmosphere of air is 4.79 and 0.0089 mol/h per square meter. The oxidation rates of 0.05-1 M V(II) electrolytes to V(III) electrolytes are approximately 96-538 times than that of V(III) electrolytes to V(IV) electrolytes.

Neural Network Predictive Control for Vanadium Redox Flow Battery

The vanadium redox flow battery (VRB) is a nonlinear system with unknown dynamics and disturbances. The flowrate of the electrolyte is an important control mechanism in the operation of a VRB system. Too low or too high flowrate is unfavorable for the safety and performance of VRB. This paper presents a neural network predictive control scheme to enhance the overall performance of the battery. A radial basis function (RBF) network is employed to approximate the dynamics of the VRB system. The genetic algorithm (GA) is used to obtain the optimum initial values of the RBF network parameters. The gradient descent algorithm is used to optimize the objective function of the predictive controller. Compared with the constant flowrate, the simulation results show that the flowrate optimized by neural network predictive controller can increase the power delivered by the battery during the discharge and decrease the power consumed during the charge.

Fe/V redox flow battery electrolyte investigation and optimization

The recently invented iron (Fe)/vanadium (V) redox flow battery (IVB) system has attracted increasing attention because of its long-term cycling stability and low-cost membrane/separator. In this paper, we describe our extensive matrix study of factors such as electrolyte composition, state of charge (SOC), and temperature that influence the stability of electrolytes in both positive and negative half-cells. During the study, an optimized electrolyte that can be operated in a temperature range from −5 °C to 50 °C without precipitation is identified. Fe/V flow cells using the optimized electrolyte and low-cost separator exhibit satisfactory cycling performance at different temperatures. Efficiencies, capacities, and energy densities of flow batteries at various temperatures are studied.

Modeling a vanadium redox flow battery system for large scale applications

A simulation model of a vanadium redox flow battery (VRFB) system based on measurements with a kilowatt scale real life VRFB unit was developed. Various hourly charging and discharging cycles were performed for states of charge (SOC) of 75%, 60%, 40% and 20% at different power values ranging from 2kW to 10kW. The dependence of the overall system efficiency in a VRFB unit on the SOC was determined by considering the energy losses at stacks during the electrochemical conversion and by the auxiliary power consumption of the hydraulic circuits as well as the power conversion systems. Using the model, optimal number of modules for certain power levels during charging and discharging operations were estimated for megawatt scale operations.

Study on Partial Degradation Operation Modes of VRB-ESS

Vanadium Redox Flow Battery (VRB) holds unique characteristic as the power and capacity of the physical structure of independent design, which determines VRB different from other battery types in operating characteristics by taking unique operation modes under the circumstance of module degradation or failure in some cases. Paper focuses on how to ride through the case of VRB system with partial degradation or failure during the normal operational conditions, launched a comprehensive experimental study and theoretical analysis. The paper analyzes several situations from the point of view of the battery physical components which should lead VRB system partial failed, and simulated the battery partial degradation or failure by experimental means, and carried out two operation modes to respond to different partial failure situations, which including the use of lower rated power and the use of times rated power. Study proved that those two modes operated in the case of VRB partial failure conditions, has no significant effect to VRB Energy Storage System (ESS) efficiency, as well as be helpful to improve a higher reliability of VRB energy storage system. This paper takes the lead in studying the operational characteristics of VRB- ESS from the angle of partial degradation or failure conditions, which has practical engineering significance for developing VRB -ESS demonstration.

Sulfonated polyimide/chitosan composite membrane for vanadium redox flow battery: Influence of the infiltration time with chitosan solution

In order to probe the effect of the infiltration time with chitosan solution, a series of sulfonated polyimide/chitosan (SPI/CS) composite membranes with different infiltration time including 6h, 12h, 24h and 36h were prepared and applied in vanadium redox flow battery (VRB) system. SEM images demonstrate the presence of thin CS layer on the surface of SPI membrane, and the thickness of the CS layer increases with the infiltration time. The proton conductivity and the vanadium ion barrier property of SPI/CS composite membranes increase with the increment of the infiltration time. The SPI/CS-24 and SPI/CS-36 membranes show similar proton selectivity which is over eight times of that of Nafion® 117 membrane. VRB single cell using SPI/CS membrane reveals higher coulombic efficiency (CE) and energy efficiency (EE) than that using Nafion® 117 membrane. In particular, the CE and EE for SPI/CS-24 membrane achieve 97.8% and 88.6% respectively. All experimental results indicate that the SPI/CS composite membranes are promising proton conducting membranes for VRB, among which SPI/CS-24 membrane exhibits the best combination property.

Design and development of unit cell and system for vanadium redox flow batteries (V-RFB)

Vanadium redox flow battery (V-RFB) has been attracted by many researches; some are under field testing and demonstration stage, but information on construction, experimental characterization, electrolyte preparation, overall systems under study, etc. are still limited. This paper focus on the technical issues faced and the lessons learnt during the development of unit cell and system for V-RFB. Firstly, brief description on problem identification, development and implementation in cell design and system for V-RFB are discussed. Secondly, preliminary experiment on 25 cm2 laboratory, unit cell V-RFB presents various difficulties such as its high tendency to fall under failure mode are presented. Finally, discussion on experimental result which shows significant improvement on V-RFB system efficiency up to 72% with reduction of contact resistance, recorded an average of 8.6 mΩ. In addition, the newly developed system provides a constructive base for future studies in temperature-controlled system and a divided, open-circuit potentiometric cell for half-cell redox analysis.

An optimal strategy of electrolyte flow rate for vanadium redox flow battery

Electrolyte flow rate is a key factor that affects the performance of vanadium redox flow battery (VRFB). A kilo-watt class VRFB system is fabricated to investigate the effects of electrolyte flow rate on the performance of VRFB. The experiments show that the capacity increases, but the system efficiency decreases with the increase of electrolyte flow rate. An optimal strategy of electrolyte flow rate is proposed to improve the system efficiency and keep the high capacity simultaneously, which is corresponding to optimize the electrolyte flow rate at different stages of charge and discharge processes. The results show that the system efficiency can be improved as high as 8% when keeping high capacity simultaneously.

A significantly improved membrane for vanadium redox flow battery

A novel sandwich-type sulfonated poly(ether ether ketone) (SPEEK)/tungstophosphoric acid (TPA)/polypropylene (PP) composite membrane for a vanadium redox flow battery (VRB) has been developed with improved properties: the permeability of vanadium ions is greatly reduced and the performance of the VRB cell is greatly increased. The membrane is based on a traditional SPEEK membrane embedded with TPA but PP is used to enhance the membrane for the first time. Although its voltage efficiency (VE) is a little lower than that of a Nafion 212 membrane, it is expected to have good prospects for VRB systems because of its low cost and good performance.

Amphoteric ion exchange membrane synthesized by radiation-induced graft copolymerization of styrene and dimethylaminoethyl methacrylate into PVDF film for vanadium redox flow battery applications

Poly(vinylidene difluoride) (PVDF) film was grafted with styrene (St) and dimethylaminoethyl methacrylate (DMAEMA) using γ-irradiation techniques. Through subsequent sulfonation and protonation processes, a new kind of amphoteric ion exchange membrane (AIEM) was synthesized. The grafting yield ( GY) increased with absorbed dose and leveled off at about 60 kGy. The composition of poly(St- co-DMAEMA) grafts was correlated to the ratio of St to DMAEMA monomer in the feed. Micro-FTIR and XPS analyses testified that the grafting and sulfonation of St unit in poly(St- co-DMAEMA) grafts had been carried out as designed. Further characterizations showed that the properties of the AIEM strongly depended on the composition and GY of the film, i.e. higher content of DMAEMA brought lower permeability of vanadium ions and conductivity, while higher GY led to higher water uptake, ion exchange capacity ( IEC) and conductivity. Finally, an AIEM with a GY of 26.1% was assembled and tested in the vanadium redox flow battery (VRFB) system. It was found that the VRFB assembled with the AIEM maintained an open circuit voltage (OCV) higher than 1.2 V after placed for 68 h, which was much longer than that with the Nafion117 membrane. Therefore, this work provides a novel method to develop potential substitute of Nafion membranes to be applied in VRFB system.

Nafion/organically modified silicate hybrids membrane for vanadium redox flow battery

In our previous work, Nafion/SiO 2 hybrid membrane was prepared via in situ sol–gel method and used for the vanadium redox flow battery (VRB) system. The VRB with modified Nafion membrane has shown great advantages over that of the VRB with Nafion membrane. In this work, a novel Nafion/organically modified silicate (ORMOSIL) hybrids membrane was prepared via in situ sol–gel reactions for mixtures of tetraethoxysilane (TEOS) and diethoxydimethylsilane (DEDMS). The primary properties of Nafion/ORMOSIL hybrids membrane were measured and compared with Nafion and Nafion/SiO 2 hybrid membrane. The permeability of vanadium ions through the Nafion/ORMOSIL hybrids membrane was measured using an UV–vis spectrophotometer. The results indicate that the hybrids membrane has a dramatic reduction in crossover of vanadium ions compared with Nafion membrane. Fourier transform infrared spectra (FT-IR) analysis of the hybrids membrane reveals that the ORMOSIL phase is well formed within hybrids membrane. Cell tests identify that the VRB with Nafion/ORMOSIL hybrids membrane presents a higher coulombic efficiency (CE) and energy efficiency (EE) compared with that of the VRB with Nafion and Nafion/SiO 2 hybrid membrane. The highest EE of the VRB with Nafion/ORMOSIL hybrids membrane is 87.4% at 20 mA cm −2, while the EE of VRB with Nafion and the EE of VRB with Nafion/SiO 2 hybrid membrane are only 73.8% and 79.9% at the same current density. The CE and EE of VRB with Nafion/ORMOSIL hybrids membrane is nearly no decay after cycling more than 100 times (60 mA cm −2), which proves the Nafion/ORMOSIL hybrids membrane possesses high chemical stability during long charge–discharge process under strong acid solutions. The self-discharge rate of the VRB with Nafion/ORMOSIL hybrids membrane is the slowest among the VRB with Nafion, Nafion/SiO 2 and Nafion/ORMOSIL membrane, which further proves the excellent vanadium ions blocking characteristic of the prepared hybrids membrane.

Preparation and characterization of Nafion/SPEEK layered composite membrane and its application in vanadium redox flow battery

In order to reduce the cost of membrane used in vanadium redox flow battery (VRB) system while keeping its chemical stability, Nafion/sulfonated poly(ether ether ketone) (SPEEK) layered composite membrane (N/S membrane) consisting of a thin layer of recast Nafion membrane and a layer of SPEEK membrane were prepared by chemical crosslink the sulfonic acid groups of different ionomer membranes. Scanning electron microscopy (SEM) and IR spectra analysis of the membrane showed that Nafion layer was successfully deposited on the SPEEK membrane surface and an integral layered membrane structure was formed. The area resistance and permeability of vanadium ions of membrane were also measured. It was found that N/S membrane have a very low permeability of vanadium ions accompanied by a little higher area resistance compared with Nafion membrane. As a result, the VRB single cell with N/S membrane exhibited higher coulombic efficiency and lower voltage efficiency compared with VRB single cell with Nafion membrane. Although N/S membrane delivered relatively lower energy efficiency compared with Nafion membrane, its good chemical stability and low cost make it a suitable substitute for Nafion membrane used in VRB system.

Image quality and use of the U.S. Army Quartermaster solar furnace

A simulation model of a vanadium redox flow battery (VRFB) system based on measurements with a kilowatt scale real life VRFB unit was developed. Various hourly charging and discharging cycles were performed for states of charge (SOC) of 75%, 60%, 40% and 20% at different power values ranging from 2 kW to 10 kW. The dependence of the overall system efficiency in a VRFB unit on the SOC was determined by considering the energy losses at stacks during the electrochemical conversion and by the auxiliary power consumption of the hydraulic circuits as well as the power conversion systems. Using the model, optimal number of modules for certain power levels during charging and discharging operations were estimated for megawatt scale operations.