Assessment of the reliability of vanadium redox flow batteries

Received: 6 April 2020

Revised: 9 July 2020

Accepted: 9 July 2020

DOI: 10.1002/eng2.12254

S H O RT C O M M U N I C AT I O N

Assessment of the reliability of vanadium-redox flow batteries Florian Reichelt1

Karsten Müller1,2,3

1 Institute of Separation Science and Technology, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany 2

Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Erlangen, Germany 3

Institute of Technical Thermodynamics, Universität Rostock, Rostock, Germany

Abstract Redox flow batteries are an interesting energy storage technology because they allow separate scaling of power and capacity. For their utilization on large scale, it is crucial to ensure reliable operation. Failure modes of elements of the system have been evaluated, both, regarding failure rate and severity of the different failures. As the main failure mode directly linked to a specific component of the redox flow technology, degradation of the membrane due to oxidation by vanadium ions has been identified. However, it is demonstrated that reliability is not solely determined by the specific electrochemistry of the technology.

Correspondence Karsten Müller, Institute of Separation Science and Technology, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Egerlandstr. 3, 91058 Erlangen, Germany.

Email: karsten.mueller@fau.de

A huge share of the overall failure rate is due to mechanical components such as pumps, valves, and sealing. Based on the findings it can be recommended to design the systems with a certain redundancy regarding cells and pumps but avoid excessive redundancy. This is crucial not only because of high CAPEX of redundant systems, but also because of the increased complexity with more valves and connections required for integrating redundant units.

KEYWORDS dependability, energy storage, failure rate, flow battery, risk management, VRFB

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I N T RO DU CT ION

Redox flow batteries (RFBs) are electrochemical flow systems that store energy in soluble redox couples and which typically permit to separate storage capacity and power output. The energy is stored in form of two liquid media containing a redox system. These liquids are pumped through a cell, where electrochemical conversion takes place. An interesting feature of RFBs is the independent scalability of capacity and power.1 Therefore, it is not necessary to have larger electrodes, if more energy has to be stored, as it would be the case for conventional batteries, where energy storage and conversion are not separated. This makes RFBs particularly interesting for large-scale storage applications in which large amounts of energy need to be stored, but the requirements regarding maximum power are moderate. The most important type of RFB is based on vanadium (the redox system V2+ /V3+ on the one side and V4+ /V5+ on the other side). Detailed descriptions of the RFB technology are reported in References 2,3. A detailed schematic can be found in Reference 4.

The work was carried out while K.M. was at Friedrich-Alexander-Universität Erlangen-Nürnberg and Forschungszentrum Jülich GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2020 The Authors. Engineering Reports published by John Wiley & Sons, Ltd. Engineering Reports. 2020;e12254. https://doi.org/10.1002/eng2.12254

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Energy storage is generally supposed to improve the dependability and resilience of energy systems. Different groups have evaluated the impact of energy storage on overall dependability of energy systems.5-7 These works line out that energy storage is suited for stabilizing energy provision by renewable sources and can improve reliability of the respective energy systems. There are also works regarding reliability of the energy storage technologies themselves; at least for most of the established storage technologies. For instance, it has been demonstrated that small degrees of redundancy in fuel cells lead to remarkable improvements of reliability.8 Other authors evaluated failures of lithium-ion batteries and concluded that failure due to deformation is caused by gas generation within the cell.9 Other works evaluated failure modes for lead-acid batteries and identified softening and shedding of positive active mass as the main cause of failure rather than water loss or sulfation.10 Yet, for more sophisticated technologies, such as RFB, knowledge on reliability is still limited.

Even though invented some decades ago, the technology is only slowly reaching larger market penetration. As a consequence, long-term experience concerning reliability of the systems is not available so far.11 Still, this type of information is crucial for both, investment decisions and improvement of the technology. Some studies exist regarding durability of individual components. Trovò et al4 report that temperature in an RFB system exceeds a critical limit of 50◌ C at the end of long discharge periods. This can lead to precipitation and in worst case to failure of the battery. Nafion-type membranes are most common in RFB applications. However, even though not widely in use, other types of membrane materials have been studied as well. Fujimoto et al studied durability of membrane materials based on sulfonated Diels-Alder poly(phenylene)s and reported significant degradation of the material.12 Degradation is mainly due to oxidation by V5+ ions. Sukkar and Skyllas-Kazacos13 performed a more comprehensive study on membrane stability. They reported that the evaluated membranes have reasonable stability but point out that extended exposure to the fully charged solution should be avoided. Liu et al14 report on corrosion of graphite electrodes in vanadium RFBs and pointed out that anodic polarization should be kept below 1.6 V. Yet, most works dealing with aspects related to reliability only evaluate materials, but only very few focus on the system level.

Tsianikas et al15 performed a system level analysis for electric grids, including an evaluation of the effect of different battery types on grid-outage resilience. According to their study, only sodium-sulfur batteries have a slightly more positive effect on grid-reliability than RFBs. Actual analysis of RFB reliability has been reported by Xiong et al in rather recent studies. They developed three-dimensional models to localize areas of high mechanical stress.16 From this analysis the authors concluded that excellent sealing is of utmost importance. Furthermore, they evaluated mechanical parameters from tensile strength tests and used these data in simulations of the stack. In this study, they concluded that a reduction in clamping force has a positive effect on failure probability.17 A different focus was followed by Chen et al,18 who evaluated strategies for RFB systems consisting of several stacks to cope with failures in some elements of the system and still provide the required power.

In their review on engineering aspects of RFBs, Arenas et al outlined the lack in current literature on failure modes and at the same time the relevance of these considerations. An engineering task that it pointed out is the necessity to prevent contamination of electrodes with corrosion products from failed stacks.19 The importance of understanding and improving reliability is also stressed by authors of other reviews on RFB engineering20,21 and energy system reliability.22 However, studies on the failure rates of RFBs on the system level have not been reported so far. This communication provides a first contribution to the assessment of reliability for RFB systems. It aims at enabling basic understanding of the most relevant failure modes in RFB systems. This is an important foundation for the further improvement of the technology toward full market maturity and into understanding the effect of RFBs on reliability on a system level. As a first step, the basics of the methodology for estimating failure rates of systems are introduced. Afterward, results on the relevance of different components of RFBs are presented and discussed. As a last step, the scope is extended over the pure evaluation of probability by including the different severity of the individual failure modes.

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M ET H O D O LO GY

As a first step, the probability of failures (or failure rate) is estimated. Quantitative values are usually reported as Mean Time to Failure (MTTF). It describes the statistical average duration a component stays functional before a failure event occurs. The MTTF values are obtained experimentally from long-time experience with a huge number of the respective component. Mathematically, the failure rate is the reciprocal of the MTTF: đ?œ†=

1 MTTF

(1)

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Analogous to the Anna Karenina principle, a system is only operational if all elements are operational at the same time.

Hence, the failure rate of a system is determined by the sum of the failure rates of its individual serial (ie, non-redundant) components: đ?œ†system =

∑

đ?œ†i

(2)

i=component

In some technologies, failure of some of the elements only leads to a loss of a certain share of functionality. For instance, if only the ability to charge or discharge, respectively, is lost, the system can still maintain a share of its functionality at least for a certain time. Loosing charging functionality, the storage can still provide energy as long as the storage reservoir is not yet emptied. This can enable emergency shutdown or provide enough time to realize an alternative energy provision.

In some cases, failure of an element does not necessarily lead to system failure, because other elements can cover the respective functionality (ie, redundancy).

For RFBs, malfunctions causing loss in one of the functionalities lead to a loss of the other functionality as well. On the other hand, RFBs (like most battery technologies) offer the possibility to implement redundancy quite easily. For example, two instead of one stack can be used for the electrochemical conversion. If one of them fails, the system is at least able to provide half of the power. However, in this study we assume a simple RFB system without redundancy to identify potential problems.

The procedure of the analysis first requires a system description consisting of a list of all elements and their number in the system. In a second step, a MTTF value is assigned to each element. To do so, literature values are used, if available. If there are no literature values available or they vary within a huge range, reasonable assumptions have to be made. Simply taking an average of the values is usually not justified, because single extreme values would distort the result. It is often more reasonable to utilize an average value of the failure rate than of MTTFs. When evaluating MTTF values it should be kept in mind that they are only rough approximations. For valves and pumps there is a comparatively good database available, but even here a huge range can be found. For RFB electrodes and similar components there are only very few data available and many assumptions have to be made.

As a last step, the MTTF values are converted into failure rates according to Equation (1). For the S-P matrix, each possible failure event is further analyzed with regard to severity. Each failure event is afterward assigned to a severity class ranging from 1 (negligible consequences) to 10 (fatal consequences highly likely). The definitions of the severity classes are reported as electronic supporting information.

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R E S U LTS AN D D ISCU SSIONS

A first step of an analysis aiming at improving reliability is the identification of the most critical elements of the technology.

Figure 1 illustrates contributions of different constituents of the system to the overall failure rate. The values have been estimated based on life times for the respective (or if not available: similar) components reported in literature (Table 1; see the supporting information for more details).

FIGURE 1

Estimated contributions of system elements to total failure rate of a flow battery

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Estimated MTTF/h Leakage on tubing

12 000

Valves

30 000

Membrane

Minor: 38344

T A B L E 1 MTTF values for the system components considered in this study

Major: 153374 Electrodes and bipolar plates

43 800

Pumps

43 800

Electrolyte

88 500

Tanks

1 100 000

Filters

1 400 000

It strikes that the two main sources for malfunction are related to aspects not specific to the RFB technology. Leakage of anolyte or catolyte liquid is a comparatively likely event. The electrolyte circuit loops have many components connected to, for example, pumps, tanks, the cell, or valves. Due to the high number of these connections, their individual failure probabilities add up to one of the highest probabilities for failures. A similar effect can be observed for the valves. The failure rate of a single valve is not very important, but since there are more valves in the system, valve failures are comparatively frequent. This is consistent to findings by RĂźde et al for energy storage using Liquid Organic Hydrogen Carrier.23 For this storage technology, valves also contribute a major share to the overall system failure rate due to their high number. For RFBs, pumps are needed to circulate the electrolyte solution. As mechanical elements, involving moving parts, pumps are also quite prone to failure. Their contribution to the system failure rate is in a similar order of magnitude than for the valves.

In case of a fault event, repair is required. The duration of this repair depends on factors, such as availability of spare parts or qualified personal. The respective repair is usually done by replacing the defective component. This requires separation from the system to avoid major leakage of electrolyte liquid or its contamination during repair.

The most critical elements specific to the RFB technology are the assembly of electrodes, bipolar plates and membrane.

The main degradation mechanism for the material of the electrode, the bipolar plate or the membranes, is oxidation by V5+ ions.14 In case of substantial corrosion, this degradation can lead to leakage. Internal leakage can open shortcuts for the fluid flow. Hence, the flow profile would become suboptimal and performance of the systems would go done. Heavy corrosion can even cause external leakage, that is, anolyte or catolyte dropping out of the system. However, the main problem with corrosion of electrodes and bipolar plates are the products of the reaction. Particles formed by corrosion can inflict damage on other system components such as pumps, valves or the membrane. Contamination with particles can also enter the system through corrosion of current collectors or leakages. To avoid subsequent damage by such contamination, an on-site purification procedure of the electrolyte can be implemented.24 For the bipolar plate a further damaging mechanism can be extrusion failure.

Slow membrane degradation can be detected in time by suited monitoring procedures. That way regular maintenance can avoid larger issues related to failing membranes. Yet, there is a certain risk of negative effects due to partially degraded membranes causing for example, enhanced crossover. Massive failure of membranes, that is, sudden major rupture, is less likely, but regarded as a separate failure mode, because it would lead to an immediate system break down. Smaller ruptures, particularly near the inlets, are much more likely. This does not lead to an immediate failure of the overall system, but still requires appropriate measures. Mechanical stress can be imposed to the membrane by particles within the electrolyte solution.25 Repairs at the assembly of electrodes, bipolar plates and membrane in most cases will be more complex than replacing a defect valve. Consequently, repair time is longer and higher qualified personal is required.

Degradation of the electrolyte solution is not known as a common problem. However, certain fouling or other changes of the solution might occur over time.

Redundancy of certain elements of a RFB can help to increase reliability. A redundant setup of the stacks can make sense. Outage of one stack leads to lower maximum power output. However, the system is still able to work at least at partial load. For this purpose, it has to be possible to isolate the damaged stack with respective valves. Redundant pumps are also an opportunity to increase systems dependability. Nevertheless, one should keep in mind that redundancy requires additional connections and valves. Hence, the effect on reliability can be partially consumed or even overcompensated. It

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is therefore important to find a reasonable compromise between redundancy and system complexity to achieve the most dependable solution. To do so, the increase in failure rate due to additional serial components has to be compared to the decrease due to parallel (ie, redundant) components for each scenario. Additional serial components can be, for example, valves for connecting and disconnecting the redundant component with/from the rest of the system. The probability of failure, even for the redundant functionalities, is still not zero. It can be calculated by multiplying the individual failure probabilities of the redundant components (eg, if a component, that is only needed once, is safeguarded by a second component of the same type, the probability of failure is squared; since probabilities are smaller than one, the resulting probability of both components failing at the same time is much lower).

Furthermore, it should be kept in mind that the most reliable system is not necessarily the economically best solution. In some cases, it can make sense to install less redundancy than technically optimal. This reduces investment costs (CAPEX), because less redundant elements, which are only required rarely for maintaining operation, are needed.

The tradeoff between redundant pumps and cells and the negative effect of additional tubing and valves should not only take the total failure rate into account, but also the severity of the different failure modes. Blocking valves or rupture of a membrane lead to a total system outage. Leakage on tubing or tanks is also a failure mode and requires repair measures.

Nonetheless, it is far less critical (at least if a suited drip pan is installed to avoid environmental issues). A valuable tool for this type of analysis is the S-P-matrix (Figure 2). In this type of diagram, the rate of occurrence of different undesired events on one of the systems components (ie, the failure rate) is plotted over the severity of this event (the severity classes and their definitions are described in more detail in the supporting information). Failure modes close to the top right are most critical, while events become less relevant as they get closer to the bottom left corner.

The analysis again outlines, that attention should be paid to mechanical elements like valves and pumps. The tubing system on the other hand is of much lower relevance than Figure 1 might suggest. The high frequency of failures is mainly due to minor leakage incidents, which occur quite frequently on connections, but do not cause major problems for system operation or safety. Membranes are also important with regard to reliability, but the severity of a leaking membrane is not greater than for a blocked valve or a failing pump. Minor damages, such as reduced retention capacity of thinner membranes due to partial dissolution, only lead to a slow loss of storage capacity. Major issues, like membrane rupture, cause a sudden loss of functionality and can cause a strong temperature increase. However, as long as aqueous solutions are utilized, there is no risk of ignition (in contrast to some conventional battery types). Thus, the probability of catastrophic damages beyond the boundaries of the RFB system itself (which might even include injuries or fatalities) is consequently low. As it has been shown for a zinc-bromide flow battery, fire hazards can still originate from electric components such as the inverters.26 The findings reported in this study extend the scope of the existing literature on RFBs and their reliability to the system level. It could be shown that particular attention should be paid to elements not considered in previous studies. A huge share of problems during operation is due to components, which are not dealing with the redox chemistry itself, but with the balance of plant.

Two aspects should be kept in mind with regard to this study. First, all reliability studies are associated with a huge margin of uncertainty and all quantifications are only rough orders of magnitude. Second, this study is supposed to be a first attempt for accessing reliability of RFBs on a system level. It aims at identifying potential weaknesses. More detailed analysis should be done in the future.

FIGURE 2

S-P-matrix representing the probability and severity of individual failures

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The methods discussed in this article aim at assessing the statistical average operation time between failures. This is important knowledge, both, for investment decisions and for identification of weaknesses to improve the technology.

During operation, online prediction methods are necessary. This type of failure prediction enables estimates of residual lifetime of components. This allows for taking measures before the fault event actually occurs. Failure prediction is a common tool for example, in computer science.27 Similar technics can also be applied for battery systems. For instance, recurrent neural networks can be used to predict the remaining useful life-time of lithium ion batteries.28 However, prediction methods for failure of RFBs and their individual system components are still a field to be addressed in further research.

4

CO N C LU S I O N

RFBs, as a promising technology for energy storage, can contribute to the overall reliability of energy systems. However, the reliability of the RFB itself should be taken into account as well. The analysis of RFB systems showed that a focus on the stack, as it is usually done in literature, is insufficient. A huge share of the overall failure probability is associated with components needed for liquid processing. This is particularly relevant in view of redundancy. Concerning the stacks, it is relatively easy to achieve a high degree of redundancy. Nevertheless, this causes further system complexity, including more pipes and valves. Failures in these additional components can overcompensate benefits regarding reliability due to redundancy of the stacks.

In this study, a first systems level evaluation of failure modes for RFBs is reported. It is an important basis for the understanding of dependability of this emerging technology and enables future works on the development of more reliable RFBs. Further research on RFB reliability should address the topic of recovery from failure. Avoiding failure is important, but for many practical applications, it is even more relevant that a dysfunctional systems resumes service quickly. Based on the findings reported here such an analysis of resilience can be performed in future works.

PEER REVIEW INFORMATION Engineering Reports thanks the anonymous reviewers for their contribution to the peer review of this work.

PEER REVIEW The peer review history for this article is available at https://publons.com/publon/10.1002/eng2.12254.

AU THOR’S NOTE The work was carried out while K.M. was at Friedrich-Alexander-Universität Erlangen-Nürnberg and Forschungszentrum Jülich GmbH. CONFLICT OF INTEREST The authors declare no potential conflict of interest.

AU THORS CONTRIBUTION Florian Reichelt contributed to the data curation, formal analysis, investigation, methodology, writing the original draft.

Karsten Müller contributed to the conceptualization, methodology, supervision, writing the original draft.

ORCID Karsten Müller

https://orcid.org/0000-0002-7205-1953

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SUPPORTING INFORMATION Additional supporting information may be found online in the Supporting Information section at the end of this article.

How to cite this article: Reichelt F, Müller K. Assessment of the reliability of vanadium-redox flow batteries.

Engineering Reports. 2020;e12254. https://doi.org/10.1002/eng2.12254