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Membrane evaluation and thermal modelling of the vanadium redox flow battery

Kronander, Anders LU (2016) KET920 20161
Chemical Engineering (M.Sc.Eng.)
Abstract
In this study, different membranes were tested for their use in a vanadium redox flow battery (VRB). The membranes were first tested under charge-discharge operation of the VRB over four charge-discharge cycles at different current densities (20 mAcm-2, 40 mA cm-2 and 60 mAcm-2) and the corresponding cell performance was evaluated in terms of total energy efficiency (EE), coulombic efficiency (CE) and voltage efficiency (VE).

The anion exchange Fumasep® series FAP-450 as well as the cation exchange series, Fumapem®, F-930 by Fumatech™ (located in Baden-Württemberg, Germany) and the GN-115, GN-212, GN-212C (all from General Energy and New Materials Co Ltd,. Nanjing, China) and the VB1 (supplied by V-Fuel Pty Ltd) were all evaluated... (More)
In this study, different membranes were tested for their use in a vanadium redox flow battery (VRB). The membranes were first tested under charge-discharge operation of the VRB over four charge-discharge cycles at different current densities (20 mAcm-2, 40 mA cm-2 and 60 mAcm-2) and the corresponding cell performance was evaluated in terms of total energy efficiency (EE), coulombic efficiency (CE) and voltage efficiency (VE).

The anion exchange Fumasep® series FAP-450 as well as the cation exchange series, Fumapem®, F-930 by Fumatech™ (located in Baden-Württemberg, Germany) and the GN-115, GN-212, GN-212C (all from General Energy and New Materials Co Ltd,. Nanjing, China) and the VB1 (supplied by V-Fuel Pty Ltd) were all evaluated during charge-discharge operation. It was found that the F930 membrane outperformed all other membranes at all current densities tested in this study with a peak average EE of 89,6% at 500 mA (20 mAcm-2) and CE of 98,8% at 1500 mA (60 mAcm-2). The second best at charge-discharge evaluation was the FAP450 with an overall EE of 81,0%.

In order to further quantify any differences between the membranes, these membranes were tested for permeability rates at constant temperature for all vanadium ions present in a VRB without influence of any current. The diffusion coefficients through the FAP-450 and the F930, VB2 (same as VB1 but thicker) and the GN-115 membrane were determined for the following ions V(II), V(III), V(IV) and V(V). It was found that the VB2 had the lowest measured permeability rates for the V(II), V(III) and V(IV) ions. Furthermore, the FAP450 membrane was the membrane with the second best diffusion coefficients which was consistent with the high coulombic efficiencies observed in the laboratory scale VRB cell.

To predict the overall capacity loss during long term operation, the battery was simulated in MATLAB™ using mathematical models developed at UNSW Australia for 60 cycles (without electrolyte remixing). It was found that the FAP450 performed better than the F930 and the GN-115 membrane based on experimentally determined diffusion coefficients. The self-discharge reactions led to almost 0% capacity loss after about 60 cycles for the FAP450 membrane. For the F930, which had about 10 times higher diffusion coefficients compared to the FAP450, about 90% capacity was lost over the same number of cycles. The GN-115 membrane, which had higher diffusion coefficients than the F930 gave better results at simulations and had lost about 50% of the capacity after 60 cycles. This is believed to be due to the F930 having a factor of 20 difference between the diffusion coefficients which leads to a buildup of vanadium ions on one side of the cell and a deficit of the other. As the F930 had overall smaller diffusion coefficients compared to the GN115, it was unexpected that the latter would prove better than the F930 at extended use. It can thus be concluded that not only do the diffusion coefficients have to be low, they also have to be the same order of magnitude for all the different ions to prevent severe capacity drop.

As the vanadium electrolytes may irreversibly precipitate at low respectively high temperatures it is imperative to understand how non-electrochemical, exothermic side reactions as well as surrounding temperatures interact with the electrolytes. Therefore, thermal modelling was also undertaken using the MATLAB model developed at UNSW. It was found that higher permeability rates gave much higher temperatures in the cell stacks and the electrolyte tanks. After 5 days, the electrolyte temperature in the tanks of the VRB using the F930 was about 30°C and increasing while the stacks reached temperatures of as high as 38°C and increasing. The FAP450, which had lower permeability rates than the F930, reached a stack temperature of only a few more degrees than the tank temperature due to the low influence of exothermic self-discharge reactions inside the cell stack. By comparison, previous thermodynamic simulation studies using Nafion showed that the temperature of the stack increased to about 40°C which indicates that external cooling should be considered when the pumps are turned off and the battery is at standby.

The GN-115 membrane from General Energy © gave good results in the VRB cycling tests as well as for permeability rates and was therefore further evaluated by immersion in 1 M V(5), 2,5 M H2SO4 for 7 weeks in order to test its chemical resistance to the oxidizing V(5) solution. It displayed a 26% increase in thickness and about 6% increase in length and width. The weight increased by about 10%. This indicates that the pore size of the membrane might have changed during immersion which could influence the performance of the membrane in VRBs. (Less)
Popular Abstract (Swedish)
Framtidens energilagring banar väg åt förnyelsebar elenergi

Andelen förnyelsebar elenergi på marknaden ökar ständigt. En negativ aspekt med dessa källor är att vi inte kan kontrollera när energin skall vara tillgänglig – vindkraften behöver vind och solkraften behöver strålande sol. För att överkomma dessa motsättningar och göra energin mer tillförlitlig behöver vind och solenergi kompletteras med enheter för storskalig energilagring för att täcka energibehovet även när det inte är strålande sol och när vinden inte blåser.

Självklart kan vi använda oss av fossila bränslen (och kärnkraft) när vår vind och solkraft inte genererar tillräckligt, men med ökad oro för växthuseffekten och påverkade ekosystem som en följd av en... (More)
Framtidens energilagring banar väg åt förnyelsebar elenergi

Andelen förnyelsebar elenergi på marknaden ökar ständigt. En negativ aspekt med dessa källor är att vi inte kan kontrollera när energin skall vara tillgänglig – vindkraften behöver vind och solkraften behöver strålande sol. För att överkomma dessa motsättningar och göra energin mer tillförlitlig behöver vind och solenergi kompletteras med enheter för storskalig energilagring för att täcka energibehovet även när det inte är strålande sol och när vinden inte blåser.

Självklart kan vi använda oss av fossila bränslen (och kärnkraft) när vår vind och solkraft inte genererar tillräckligt, men med ökad oro för växthuseffekten och påverkade ekosystem som en följd av en explosionsartad användning av just dessa bränslen är målsättningen att minska användningen dramatiskt.

Hur kan vi lagra energi?

Ur ett förnyelsebart perspektiv (alltså utanför den energi som finns lagrad i stenkol och andra fossila bränslen) utgör vattenkraft 98% av den totala lagringskapaciteten världen över. I Sverige står vattenkraften för ungefär 50% av vårt totala behov av elenergi. Vattenkraft är tillförlitligt, storskaligt och bidrar inte till några utsläpp av växthusgaser, men kräver synnerligen gynnsamma naturliga förhållanden för att kunna bli aktuell. Dessutom är vattenkraften till stor del utbyggd till maximal kapacitet världen över och kommer inte på tal i platta länder, såsom för Belgien och Nederländerna, där en annan typ av energilagring krävs för att etablera användningen av vind och solkraft ytterligare. Lösningen kan ligga i en ny typ av lagringsenheter, så kallade flödesbatterier, som kan återanvändas i miljontals cykler utan att degenereras och slitas ut. Dessutom har flödesbatterierna, teoretiskt sett, ingen begränsning i kapacitet och kan därmed täcka en mängd olika behov.

Flödesbatterier

Flödesbatterier lagrar elektrisk energi som kemiskt bunden energi vid uppladdning och förvandlar kemisk energi till elektrisk energi vid urladdning. Inuti batteriet är elektrolyterna lösta i svavelsyra och vatten och pumpas omkring i systemet, vilket gett upphov till namnet. För att utöka kapaciteten ökas mängden elektrolyt. Det finns olika typer av flödesbatterier som baseras på olika aktiva ämnen, men några av de främsta, ur energieffektivitet och miljöhänseende, är de som baseras på metallen vanadin. Metallen har speciella kemiska egenskaper som gör att dessa typer av batterier kan användas under väldigt lång tid vilket, i kombination med att elektrolyten inte är giftig, ger miljövänliga egenskaper.

Andra batterier, som sådana baserade på litium eller bly, består ofta av ämnen som är giftiga och miljöfarliga plus att det kan föreligga risk för explosioner vid antändning. Vidare tappar dessa batterier ofta kapacitet efter ett antal cykler, vilket är en av många anledningar till att vi inte lagrar stora mängder energi i den typen av batterier i våra hus och hem. Flödesbatterierna tappar inte kapacitet utan kan enkelt återställas fullständigt och kan således användas under mycket lång tid.

Anledningen till att flödesbatterier ännu inte slagit igenom är på grund av kostnaden som till stor del beror på membranet som skiljer elektrolyterna från varandra. Genom detta examensarbete undersöktes därför potentiella, billigare membran med hjälp av experiment och datoriserade simuleringar. Genom dessa studier isolerades två lovande kandidater för vidare användning i batteriet som kan komma att sänka kostnaderna rejält. (Less)
Please use this url to cite or link to this publication:
author
Kronander, Anders LU
supervisor
organization
course
KET920 20161
year
type
H2 - Master's Degree (Two Years)
subject
keywords
thermodynamics, Thermal, Simulation, Energy storage solutions, Modelling, VRB, Vanadium, Battery, chemical engineering, kemiteknik
language
English
id
8893837
date added to LUP
2016-11-15 14:14:55
date last changed
2016-11-15 14:14:55
@misc{8893837,
  abstract     = {In this study, different membranes were tested for their use in a vanadium redox flow battery (VRB). The membranes were first tested under charge-discharge operation of the VRB over four charge-discharge cycles at different current densities (20 mAcm-2, 40 mA cm-2 and 60 mAcm-2) and the corresponding cell performance was evaluated in terms of total energy efficiency (EE), coulombic efficiency (CE) and voltage efficiency (VE). 

The anion exchange Fumasep® series FAP-450 as well as the cation exchange series, Fumapem®, F-930 by Fumatech™ (located in Baden-Württemberg, Germany) and the GN-115, GN-212, GN-212C (all from General Energy and New Materials Co Ltd,. Nanjing, China) and the VB1 (supplied by V-Fuel Pty Ltd) were all evaluated during charge-discharge operation. It was found that the F930 membrane outperformed all other membranes at all current densities tested in this study with a peak average EE of 89,6% at 500 mA (20 mAcm-2) and CE of 98,8% at 1500 mA (60 mAcm-2). The second best at charge-discharge evaluation was the FAP450 with an overall EE of 81,0%. 

In order to further quantify any differences between the membranes, these membranes were tested for permeability rates at constant temperature for all vanadium ions present in a VRB without influence of any current. The diffusion coefficients through the FAP-450 and the F930, VB2 (same as VB1 but thicker) and the GN-115 membrane were determined for the following ions V(II), V(III), V(IV) and V(V). It was found that the VB2 had the lowest measured permeability rates for the V(II), V(III) and V(IV) ions. Furthermore, the FAP450 membrane was the membrane with the second best diffusion coefficients which was consistent with the high coulombic efficiencies observed in the laboratory scale VRB cell. 

To predict the overall capacity loss during long term operation, the battery was simulated in MATLAB™ using mathematical models developed at UNSW Australia for 60 cycles (without electrolyte remixing). It was found that the FAP450 performed better than the F930 and the GN-115 membrane based on experimentally determined diffusion coefficients. The self-discharge reactions led to almost 0% capacity loss after about 60 cycles for the FAP450 membrane. For the F930, which had about 10 times higher diffusion coefficients compared to the FAP450, about 90% capacity was lost over the same number of cycles. The GN-115 membrane, which had higher diffusion coefficients than the F930 gave better results at simulations and had lost about 50% of the capacity after 60 cycles. This is believed to be due to the F930 having a factor of 20 difference between the diffusion coefficients which leads to a buildup of vanadium ions on one side of the cell and a deficit of the other. As the F930 had overall smaller diffusion coefficients compared to the GN115, it was unexpected that the latter would prove better than the F930 at extended use. It can thus be concluded that not only do the diffusion coefficients have to be low, they also have to be the same order of magnitude for all the different ions to prevent severe capacity drop. 
 
As the vanadium electrolytes may irreversibly precipitate at low respectively high temperatures it is imperative to understand how non-electrochemical, exothermic side reactions as well as surrounding temperatures interact with the electrolytes. Therefore, thermal modelling was also undertaken using the MATLAB model developed at UNSW. It was found that higher permeability rates gave much higher temperatures in the cell stacks and the electrolyte tanks. After 5 days, the electrolyte temperature in the tanks of the VRB using the F930 was about 30°C and increasing while the stacks reached temperatures of as high as 38°C and increasing. The FAP450, which had lower permeability rates than the F930, reached a stack temperature of only a few more degrees than the tank temperature due to the low influence of exothermic self-discharge reactions inside the cell stack. By comparison, previous thermodynamic simulation studies using Nafion showed that the temperature of the stack increased to about 40°C which indicates that external cooling should be considered when the pumps are turned off and the battery is at standby. 

The GN-115 membrane from General Energy © gave good results in the VRB cycling tests as well as for permeability rates and was therefore further evaluated by immersion in 1 M V(5), 2,5 M H2SO4 for 7 weeks in order to test its chemical resistance to the oxidizing V(5) solution. It displayed a 26% increase in thickness and about 6% increase in length and width. The weight increased by about 10%. This indicates that the pore size of the membrane might have changed during immersion which could influence the performance of the membrane in VRBs.},
  author       = {Kronander, Anders},
  keyword      = {thermodynamics,Thermal,Simulation,Energy storage solutions,Modelling,VRB,Vanadium,Battery,chemical engineering,kemiteknik},
  language     = {eng},
  note         = {Student Paper},
  title        = {Membrane evaluation and thermal modelling of the vanadium redox flow battery},
  year         = {2016},
}