Vanadium Redox Flow Battery System Research Articles

Energy efficiency analysis for a kilo-watt class vanadium redox flow battery system

A new methodology for estimation of the key characteristics of commercial scale Vanadium Redox Flow Battery (VRFB) at different operating conditions is proposed. The method is based on a set of simplified correlations that allow estimating VRFB rated power, capacity and operation time directly from the geometry of stack and tank without detailed numerical simulation of the battery. The study is focused on investigation of a kilo-watt class VRFB system (5 kW/15kWh) considering a wide range of current densities (40–100 mA cm−2). The proposed simplified approach is validated considering the most representative cases of battery operation strategies related to slow and fast modes. It demonstrated high accuracy for the estimation of rated power and operation time (average error below 3%) as well as stored energy (average error below 6%) compare to results of detailed numerical simulation. As a result, the proposed methodology can be used as a simple tool for development of proper battery usage protocol (a schedule for battery usage), which could allow avoiding over/underestimation of committed battery energy and power during battery operation. In addition, the obtained results can be also used in order to improve the accuracy of techno-economic studies determining the most economically attractive cases for application of VRFB systems.

Novel Metal Oxide Electrode Materials for Vanadium Redox Flow Battery Application

Solar cell and wind power plant have been widely used in the earth as a renewable energy source. However, those alternative energy sources have one critical problem which is the energy output cannot provide stable electricity power for common use. Since these issues, scientists have developed energy storage technique in the past several decades, such as pumped hydropower, compressed air energy storage, lithium-ion batteries, and lead-acid batteries. Recently, vanadium redox flow battery (VRFBs) is the energy storage system which can provide larger energy capacity and longer lifetime. Basically, VRFBs system can be divided by three main parts which include an electrode, membrane, and electrolyte. Nowadays, the big challenge of the carbon-based electrode has lower energy conversion efficiency. Therefore, our studies are more focusing on the electrode material development. Carbon material modification by oxygen-containing functional group or metal oxide loading has been developed in the past years. Especially, tungsten oxide nanowire shows good energy efficiency 67.7 % in higher current density 160 mA cm-2 and still remain 79 % in the energy efficiency of 80 mA cm-2 after 50 cycles test.

In-Situ Electrode Potential Measurements in Vanadium Redox Flow Batteries

Vanadium redox flow battery (VRFB) cells typically consist of a proton-conducting membrane sandwiched by two porous carbon-fiber based electrodes. The V2+/V3+ and V4+/V5+ redox couples, dissolved in acidic electrolytes, flow through the negative and positive electrodes, respectively. And, the primary reactions during charge and discharge are the redox reactions of these two vanadium couples. The round-trip efficiency (RTE) of a VRFB cell is a complex combination of multiple factors, including the electrodes, membrane, electrolytes, and cell-design configuration, as well as the operating conditions, i.e. charge/discharge current, range of the electrolyte state-of-charge (SOC) and electrolyte flow rates.1,2 The ohmic losses in the membrane and overpotentials on the two electrodes are two major factors that contribute to the cell’s efficiency losses. Undesired side reactions, including hydrogen evolution on the negative electrode and carbon corrosion on the positive electrode, may also occur, especially when the cell is overcharged, which may lead to a change in the electrolyte average vanadium oxidation state and degradation of the carbon electrodes and bipolar plates.3-5 Since both the vanadium redox reactions and these side reactions are strongly potential driven, it is useful to have a reliable reference electrode integrated into the VRFB system so that the electrode potentials vs. reversible hydrogen electrode (RHE) can be measured under in-situ conditions. The use of reference electrodes, such as the dynamic hydrogen electrode, have been reported in VRFB cells to measure in-situ electrode potentials.6,7 However, a significant challenge is the vanadium ion contamination that shifts the RHE potential higher, up to ~ 0.3V by 1.6M vanadium electrolyte. Here we describe a specially designed reference-electrode apparatus (REA), which is depicted in Fig.1 and consists of a small VRFB cell, a RHE, and a continuously acid-washed region between the VRFB cell and the RHE to prevent vanadium contamination of the RHE. This unique REA enables a reliable RHE for a VRFB cell operating under various operating conditions. For example, the REA can be used to measure both the overpotentials and electrochemical potentials (vs. RHE) of the working electrodes in a VRFB cell during a complete charge and discharge cycle, as shown in Fig. 2. The REA also provides real-time measurement of the electrolyte OCV, which can be used to determine the SOC of the posolyte and negolyte individually. The REA can also be used to provide insights into VRFB-electrolyte rebalancing options. Therefore, the REA that will be introduced here can be a useful tool to guide the selection of both electrode materials and the optimization of VRFB operating protocols, and examples of both will be presented. Acknowledgements This work was made possible by financial supported from Vionx Energy.

A model study on effects of vanadium ion diffusion through ion exchange membranes in a non-aqueous redox flow battery

A simple charge-discharge model of a nonaqueous vanadium redox flow battery (VRFB) was introduced in this study to analyze the diffusion coefficients of active materials permeating an electrolyte membrane. Using this model, the effects of diffusion coefficients on the long-term performance, efficiency, and capacity of the system have been studied. Based on a literature review, the diffusion coefficients of approximately 10−7–10−6 cm2 s−1 were applied and the results were analyzed to determine relevant parameters for this study. Through this study, it has been confirmed that the capacity of the system was maintained at 100% under ideal conditions in the long-term operation, whereas that of the system reduced by more than half when only one vanadium species moved and is 10 mAh when all vanadium species moved. In addition, the simulation clarified the effect of diffusion coefficients on coulombic efficiency. In most conditions of the various diffusion coefficients, coulombic efficiency enhanced through cycle had been operated. In the case of voltage efficiency, the value was not largely changed regardless of diffusion coefficients since it was affected more by other factors. One important factor revealed through this model study is that electrolyte membrane performance of the VRFB system was not only affected by the diffusion coefficient of a specific ion, but also by the relationship between the diffusion coefficients of all active ions involved in the reaction. This study provides guidance for the development of an electrolyte membrane to improve the performance of VRFB systems.

Correlations of Through-Plane Cell Voltage Losses, Imbalance of Electrolytes, and Energy Storage Efficiency of a Vanadium Redox Flow Battery.

Correlations of the through-plane voltage losses in the vanadium redox flow battery (VRFB), changes in the posolyte (positive electrolyte) and negolyte (negative electrolyte) conditions, energy storage capacity, and coulomb efficiency were investigated under an operando charge-discharge process. Collective and quantitative in situ studies with Ag/AgCl reference-electrodes inserted into a VRFB single-cell and the state of charge measuring external open-circuit voltage half-cells revealed that the overall performance of a VRFB is predominantly governed by (i) relatively high internal resistance of the anode half-cell and (ii) conditions of the negolyte as the limiting electrolyte. The studies showed that the round-trip coulomb efficiency, which is commonly used for VRFB performance analysis, has limitations in expressing the energy storage efficiency and capacity decay phenomena under a consecutive charge-discharge process. In situ and concurrent analysis of the internal cell overpotential and the electrolytes condition can provide quantitative information on the relative contributions of the coulombic round-trip loss, concentration loss, vanadium crossover, and charge imbalance between the posolyte and negolyte for the overall efficiency and capacity decay phenomena. This operando analysis approach is highly effective for gaining an in-depth and collective understanding of the complex phenomena taking place in the VRFB system.

On charge percolation in slurry electrodes used in vanadium redox flow batteries

In vanadium redox flow battery systems porous carbon felts are commonly employed as electrodes inside the flow channel. Recently, slurry electrodes (or flow suspension electrodes) were introduced as a potentially viable electrode system. Such electrode systems are little understood so far. Mass, momentum and charge transfer phenomena co-occur, interactions with each other are nearly impossible to capture experimentally. We present a novel discrete model of the particulate phase combining theories from fluid dynamics, colloidal physics, and electrochemistry with a coupled CFD-DEM approach. The methodology allows to visualize local phenomena occurring during the charging of the battery and to compute the net current of the slurry electrode system. We demonstrate that an increasing particle volume fraction enables the formation of conducting networks in the flow electrode until a threshold is reached. Our study concludes, that the assumption of all particles participating in the charge transfer as assumed in pure CFD investigations is not necessarily valid.

Evaluation of Grid-Connected Micro-Grid Operational Strategies

Abstract This study investigates the operational performances of a grid-connected microgrid with integrated solar photovoltaic and battery energy storage. The study is based upon the techno-economic specifications and theoretical performance of the distributed energy resource and storage systems, as well as on measured consumer load data and electrical utility retail and distribution data for representative residential and commercial loads for the city of Vasteras, Sweden. The open-source Matlab®-based simulation tool, OptiCE, is used for performing simulations and optimization. To support the attainment of one of the objectives, peak shaving of the consumer load, a battery operational strategy algorithm has been developed to balance peak shaving and PV self-consumption. Comparisons among three types of battery, lead-acid, lithium-ion and vanadium-redox flow, are also performed. A 117 kWp photovoltaic system paired with a lithium-ion battery of 41.1 kWh capacity is the optimal solution found for the considered commercial load. The calculated battery capacity represents the best trade-off for the set multi-objective optimization problem. The simulation of this system predicts the possibility to shave the customer load profile peaks up to 20% for the month of April. The corresponding self-consumption ratio is 88%. Differences in the relationship between the load profiles and the system performance have been qualitatively noted. Furthermore, the simulation results for lead-acid, lithium-ion and vanadium-redox flow battery systems reveal that lithium-ion batteries delivers the best trade-off between total annualized cost and peak shaving performance for both residential and commercial applications.

Recent development of membrane for vanadium redox flow battery applications: A review

Abstract Responding to rapid growth of the renewable energy applications, it is crucial to develop low cost and high efficient large-scale energy storage systems in order to smooth out the intermittency of the renewable energy resources. As one of the most promising large-scale energy storage systems, vanadium redox flow battery (VRFB) has attracted great attention in recent times. Membrane is one of the key components of VRFB which not only affects the whole cyclability performance but also determines the economic viability of the system. The membrane separates the positive and negative half-cells and prevents the cross-mixing of vanadium ions while providing required ionic conductivity. The ideal membrane should have good ionic exchange capacity; high ionic conductivity, low water uptake, swelling ratio, area electrical resistance and vanadium and other poly-halide ions permeability; and good chemical stability, as well as low cost. Numerous efforts have been spent on the development of different types of membranes, including different functional groups ion exchange membrane and non-ionic porous membrane. This paper reviews the research on membranes in VRFB system, including the properties, development of traditional commercial membranes as well as recently developed membranes. It explores various methods of fabrication of the membrane products which have received relatively little attention. A detailed summary table of the new membranes with their properties, fabrication and costs is provided to serve as a reference guide for researchers and industrialists interested in VRFB system building and dynamic modelling set. Subsequently, the challenges and future directions of membrane research are examined.

An Enhanced Equivalent Circuit Model of Vanadium Redox Flow Battery Energy Storage Systems Considering Thermal Effects

Thermal issue is one of the major concerns for safe, reliable, and efficient operation of the vanadium redox flow battery (VRB) energy storage systems. During the design of the operational strategy for a grid-connected VRB system, a suitable mathematical model is needed to predict the dynamic behaviors under various operating conditions. However, conventional VRB models usually neglect the impact of temperature variations on system performance. This work develops an enhanced VRB model with the consideration of the coupling effects between the electrochemical and the thermal behaviors. The proposed model consists of two equivalent circuits. First, the electrochemical behaviors of the VRB are modeled by a second-order RC network taking account of the effects of concentration variation of the vanadium ions and the electrochemical activation. Second, a third-order Cauer network is used to model the heat transfer process in the VRB system, and the dynamic thermal behaviors of stacks, pipes and heat exchangers are characterized. Well-designed experiments and particle swarm optimization algorithm are use to identify the parametric values of the developed model. The proposed modeling method was validated experimentally using a 5kW/3kWh VRB platform, and the results show that the model is capable of accurately predicting the VRB performance under variable temperature conditions. The developed coupled electro-thermal model is then used for simulating and analyzing the performance of a VRB system operated in conjunction with a wind power plant under real-world conditions.

Impact of Corrosion Conditions on Carbon Paper Electrode Morphology and the Performance of a Vanadium Redox Flow Battery

The side reactions during long-term operation of vanadium redox flow batteries (VRFBs) increase the average oxidation state of the electrolyte and associated equilibrium potential of the positive half-cell. This consequently initiates the corrosion reactions at the positive side as the half-cell potential passes the critical limit. In this study, an ex-situ accelerated electrochemical corrosion protocol is performed on the carbon paper electrode to investigate the effects of corrosion conditions induced by extended cycling on electrode morphology and VRFB system performance. In terms of morphological changes of corroded electrodes, only minor mechanical degradation is observed, including disappearance of binder material and reduced mechanical properties. With regards to the cell performance, the flow cell with the corroded electrode demonstrates notably higher charge and discharge capacities which can be attributed to the enhanced active surface area of the electrode. Furthermore, the corroded case exhibits an improved capacity retention during extended cycling which can be related to the improved redox activity caused by the increased carboxyl groups and wettability. This study shows that even at these fairly aggressive corrosion conditions, the process behaves as a treatment method, which oxidizes surface functional groups, increases active surface area, and hydrophilicity, and subsequently enhances VRFB performance.

SPES/PVDF Binary Membrane as an Alternative Proton Exchange Membrane in Vanadium Redox Flow Battery Application

SPES/PVDF blends were employed to prepare the ion exchange membranes for vanadium redox flow battery (VRB) application for the first time. The addition of the highly crystalline and hydrophobic PVDF effectively limited the swelling behavior of SPES. The vanadium ion permeability of SPES/PVDF membranes was one order of magnitude lower than that of Nafion117 membrane and pristine SPES membrane. Single cells with SPES/PVDF composite membranes showed significantly lower capacity loss, higher coulombic efficiency and higher energy efficiency than that with Nafion117 and pristine SPES membranes. The blend membrane with 40wt% of PVDF (denoted as S0.6P0.4) showed energy efficiency of 83.2% at 30 mA∙cm-2, which was superior to that of the Nafion117 and SPES membranes. In the self-discharge test, S0.6P0.4 membrane showed twice longer duration in open circuit decay than that with Nafion117 membrane. With all the good properties and low cost, the SPES/PVDF membranes are expected to have excellent commercial prospects as ion exchange membranes for VRB system.

Polybenzimidazole membranes embedded with ionic liquids for use in high proton selectivity vanadium redox flow batteries

High proton selectivity is essential for the use of ion exchange membranes (IEMs) in vanadium redox flow battery (VRFB) systems. Herein, a series of poly (oxyphenylene benzimidazole) (OPBI)/ionic liquid (IL) composite membranes was prepared by mixing OPBI with 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm]BF4) to obtain membranes with high proton selectivity. Because ILs formed hydrogen bonds with the polymer chains, ILs were able to remain within the membranes themselves. Proton conductivity and vanadium permeability of the membranes were examined for membrane characterization. Surprisingly, because of the Donnan exclusion effect between the cations of the ILs and the vanadium ions, the trade-off effect of traditional IEM modification methods was overcome. Therefore, as the IL content increased, the vanadium resistance and proton conductivity of the composite membranes increased. The proton selectivity of optimized OPBI/BF4-20 composite membrane was 1.41 × 106 S min cm−3, which was much higher than that of the unmodified OPBI membrane (6.06 × 105 S min cm−3) or a commercialized Nafion 115 membrane (1.61 × 104 S min cm−3). More importantly, the OPBI/BF4-20 composite membrane exhibited higher coulombic efficiency (CE, 99.24%), voltage efficiency (VE, 93.10%) and energy efficiency (EE, 92.39%) at 40 mA cm−2 than the unmodified OPBI (CE 98.06%, VE 90.67% and EE 88.86%) and Nafion 115 membranes (CE 95.44%, VE 91.75% and EE 87.57%). Furthermore, the cell employing the OPBI/BF4-20 membrane exhibited no significant decrease in battery efficiency after a 1000-time cycles test. In conclusion, the research and application of IL-embedded composite membranes in a VRFB system could potentially change the limitations of traditional modification methods.

Water crossover phenomena in all-vanadium redox flow batteries

Water crossover through the membrane of a vanadium redox flow battery system is not desirable because it floods one half-cell, diluting the vanadium solution on one side and consequently increasing the concentration of vanadium in the other half-cell. To analyze the effect of water crossover and the resultant electrolyte imbalance issue in the vanadium redox flow battery, herein we newly develop a water transport model and incorporate it into our previously developed 3D vanadium redox flow battery model. The model rigorously accounts for water production/consumption by the redox reaction of VO2+/VO2 and side reactions as well as various mechanisms of water crossover through the membrane arising from diffusion, electro-osmotic drag (EOD), and vanadium crossover. The numerical model is successfully validated against in situ data collected during experiments in which the electrolyte volumes and cell voltages are measured during charge–discharge cycles carried out under various current densities. The detailed simulation results clearly elucidate water crossover behaviors at different stages of charging and discharging, and further reveal the individual contributions of water crossover mechanisms to the overall electrolyte imbalance between the negative and positive sides of the VRFB system.

Ion Selectivity and Stability Enhancement of SPEEK/Lignin Membrane for Vanadium Redox Flow Battery: The Degree of Sulfonation Effect

A membrane of high ion selectivity, high stability, and low cost is desirable for vanadium redox flow battery (VRB). In this study, a composite membrane is formed by blending the sulfonated poly (ether ether ketone) with lignin (SPEEK/lignin), and optimized by tailoring the degree of sulfonation. The incorporation of lignin into the SPEEK matrix provides more proton transport pathway and meanwhile adjusts the water channel to repulse vanadium ions. The VRB cells assembled with the composite membranes exhibit high coulombic efficiency (~99.27%) and impressive energy efficiency (~82.75%). The cells maintain a discharge capacity of ~95% after 100 cycles and ~85% after 200 cycles at 120 mA cm−2, much higher than the commercial Nafion 212. The SPEEK/lignin composite membranes are promising for application in VRB system.

Ionic transportation and chemical stability of high-endurance porous polyethylene separator for vanadium redox flow batteries

To confirm the viability of a porous polyethylene (PE) separator for using in vanadium redox flow batteries (VRFBs), we conduct a comparative electroanalytical and chemical stability studies of the PE separator and Nafion 212 membrane. We characterize the physicochemical properties of the separator, such as water uptake, dimensional change, and ion conductivity, and analyze its structural and compositional features using thermogravimetric analysis, scanning electron microscopy, and X-ray photoelectron spectroscopy. We also test the chemical stability of the separator against highly oxidative V5+ ions and the cell performance of VRFB using PE separator to assess practical applicability. In chemical stability, the change of VO2+ ion to VO2+ ion is considerably lowered by the PE separator (0.01 mmol/L) than the Nafion 212 (0.27 mmol/L), which indicates that the PE separator possesses higher chemical stability. The energy efficiency of the VRFB with the PE separator is lower than that obtained with Nafion 212. However, the chemical stability of PE separator is 27 times higher than that of Nafion 212, indicating that its use will promote the long-term operation of the VRFB system. Hence, the PE separator can be considered a cost-effective option for VRFB operation, with appropriate modifications to its thickness, surface properties and pore structure.

Variable Porous Electrode Compression for Redox Flow Battery Systems

Vanadium redox flow batteries (VRFBs) offer great promise as a safe, cost effective means of storing electrical energy on a large scale and will certainly have a part to play in the global transition to renewable energy. To unlock the full potential of VRFB systems, however, it is necessary to improve their power density. Unconventional stack design shows encouraging possibilities as a means to that end. Presented here is the novel concept of variable porous electrode compression, which simulations have shown to deliver a one third increase in minimum limiting current density together with a lower pressure drop when compared to standard uniform compression cell designs.

Preparation of dense polybenzimidazole proton exchange membranes with different basicity and flexibility for vanadium redox flow battery applications

Acid doped polybenzimidazoles (PBI) membrane is a type of promising proton exchange membrane for vanadium redox flow battery (VRFB) applications and increasing the basicity of PBI is an effective way to develop the acid doped level and proton conductivity. While according to Grotthuss mechanism, the proton conduct in PBI membranes can occur from protonated guest molecules to neighbor non-protonated neighbor molecules. Therefore, the flexibility of the polymer also can influence the proton conductivity of PBI membranes. In order to investigate in VRFB system which factor is more obvious, in this article, three PBI compounds with different basicity and flexibility were synthesized and then used to prepare dense proton exchange membranes for VRFB applications. The physicochemical properties and single cell performances of these membranes were valued. The results indicated that in VRFB system, the most flexible OPBI membranes show the highest proton conductivity. The VRFB used OPBI membrane as proton exchange membrane exhibited excellent cell performance, the energy efficiency (EE) of OPBI membrane reached 81.87% at current density of 100 mA cm−2, which was obviously higher than that of a commercial Nafion 115 membrane (EE = 77.69%). The self-discharge time of the OPBI membrane was recorded to be as long as 487 h, obviously longer than Nafion 115 (56 h).

Online monitoring of state of charge and capacity loss for vanadium redox flow battery based on autoregressive exogenous modeling

Accurate monitoring of state of charge (SOC) and capacity loss is critical for the management of vanadium redox flow battery (VRB) system. This paper proposes a novel autoregressive exogenous model for the vanadium redox flow battery, based on which the model-based monitoring of state of charge and capacity loss is investigated. The offline parameterization based on genetic algorithm and the online parameterization based on recursive least squares are investigated for the proposed model to compare the model accuracy and robustness. Leveraging the parameterized model, an H-infinity observer is exploited to estimate the battery state of charge and capacity in real time. Experimental results suggest that the proposed autoregressive exogenous model can accurately simulate the dynamic behavior of vanadium redox flow battery. Compared with the offline model based method, the observer based on online adaptive model is superior in terms of the accuracy of modeling, state of charge estimation and capacity loss monitoring. The proposed method is also verified with high robustness to the uncertain algorithmic initialization, electrolyte imbalance, and the change of system design and work conditions.

Structure and Properties of Sulfonated Poly(ether ether ketone) Hybrid Membrane with Polyaniline‐Chains‐Modified Graphene Oxide and Its Application for Vanadium Redox Flow Battery

AbstractA series of acid‐base hybrid membranes (SPEEK/PANi‐GO), composed of sulfonated poly(ether ether ketone) (SPEEK) and polyaniline‐functionalized graphene oxide (PANi‐GO), have been prepared and applied for vanadium redox flow battery (VRB). The chemical composition and structure for PANi‐GO nanosheets and SPEEK/PANi‐GO hybrid membranes are in detail characterized. Physicochemical properties such as ion exchange capacity, proton conductivity and vanadium ion permeability are highly based on the loadings of PANi‐GO nanofillers. The formed acid‐base pairs, originated from ‐SO3H of SPEEK and ‐NH group of PANi‐GO, decrease the swelling ratio and vanadium ion permeability. Ion selectivity shows a strong dependence upon the PANi‐GO content, and it presents a maximum value at 2% loading. VRB performance by SPEEK/PANi‐GO‐2 hybrid membrane exhibits a higher coulombic efficiency (CE: 98.5%) and energy efficiency (EE: 81.7%) at 30 mA cm−2, as compared with Nafion 117 (CE: 88.9% and EE: 72.5%) and SPEEK membrane (CE: 89.8% and EE: 69%). The self‐discharge time with SPEEK/PANi‐GO‐2 (58 h) is longer than that of Nafion 117 membrane (26 h), and a good chemical and oxidation stability is well demonstrated after 300 times charge‐discharge cycles for SPEEK/PANi‐GO‐2 hybrid membrane. These SPEEK/PANi‐GO hybrid membranes demonstrate a promising application for VRB system.

Performance Analysis of a 5 Kw/30 Kwh Residential Vanadium Redox Flow Battery System in Denmark

The first residential vanadium redox flow battery system in Scandinavia was installed in Gladsaxe by Aarhus University and VisBlue Aps with support from the Energy Technology Development and Demonstration Program (EUDP) of Denmark. The flow battery has a nominal power output and capacity of 5 kW and 30 kWh respectively and is coupled to a 50 kW peak photovoltaic array. This solar plus storage system is integrated in a ten apartments building that already complies with the Danish Nearly Zero-Energy Building (NZEB 2020) energy performance requirements. In this work, the performance of the vanadium redox flow battery system is reported for two periods, the first between February-December 2017 and the second between February-August 2018. Performance for different periods is analyzed based on the energy balance provided by the battery inverter and the data acquired from the flow battery. The energy balance is also used to map the consumer behavior in order to develop: 1) seasonal flow battery control strategies and 2) battery power and capacity optimal sizing methods. Since the flow battery operated intermittently during 2017 before final commissioning in February 2018, the photovoltaic array was coupled to a 7.4 kW lead-acid battery pack during specific periods. An overview of the lad-acid battery performance during these periods is also reported. Finally, several technical challenges including coupling the vanadium redox flow battery with the battery inverter, stack leaks and remote monitoring of critical operating parameters are briefly discussed.