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UnderstandingtheRedflowBattery

Introduction

Flow batteries are a well understood class of energy storage technology. To date,

production flow batteries have been relatively large in terms of both physical size and

storagecapacity.Redflowhassuccessfullydesignedandcommercialisedasmall,modular,

flow battery based on zinc-bromine reactants — referred to as a ZBM2 (Zinc-Bromine

Module).Redflow’s10kilowatt-hour (kWh)ZBM2batteryhasprovedveryversatileand is

successfullyusedforavarietyofenergystorageapplications.ThisWhitePaperdescribesthe

Redflowzinc-bromineflowbatterytechnologytohighlighttheadvantagesofflowbatteries,

including their resilience and robustness. After introducing the battery, it discusses the

underlyingcompositionandlistskeyadvantagesofthistechnology.Thispaperalsooutlines

Redflow’sdesignchoicesthatmadetheZBM2batteryareliableandrobustenergystorage

system with unique advantages over legacy battery chemistries including lead-acid and

lithium-based batteries. The paper concludes by broadly addressing the topic of ZBM2

batteryintegrationandinstallation,whilenotingthatspecificapplicationnotesanddesign

guidelinescanbefoundontheRedflowwebsitewww.redflow.com.

TheRedflowZBM2ataGlance

From a strictly electrical point of view, Redflow’s ZBM2 battery presents very good

characteristics. Initsbaseconfiguration,addressedasZBM2,it isa220ampere-hour(Ah),

48-volt(V)nominalbattery-anominalenergystorageanddeliverycapacityof10kWhata

typical roundtripefficiencyof80per cent -witha voltage rangedbetween40Vand58V.

Withnoageingeffectcausedbyusage,theZBM2canbechargedanddischargedcompletely

atratesupto3kW(kilowatts)andoperatedinawiderangeofambienttemperaturesifthe

electrolyte temperature remains between 10°C and 50°C. Beyond these limits, the ZBM2

automaticallysuspendsoperationtoprotectitself.Theroundtripefficiencyofthebatteryis

around80percentwithperformancewarrantedforaminimumof3650cyclesat100per

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centdepthofdischarge(DoD)foratotalof36.5MWhofenergythroughputor

for10years–whichevercomesfirst.

Asaflowbattery,theRedflowZBM2containsabout100litresofawater-basedsolutionof

zinc-bromide salt, flowing around in two separate hydraulic circuits. During the battery

charge phase, zinc is extracted from the liquid phase and plated onto carbon-based

electrodes. During the battery’s discharge phase, zinc is ‘de-plated’ and restored to the

solution. The ZBM2 is managed entirely by an on-board Module Management System

(MMS) whilemultiple ZBMs aremanaged by Redflow’s proprietary BatteryManagement

System (BMS). The MMS controls battery operations and protects the battery against

misuse. The MMS also provides access to the battery status, real-time data, event log,

alarms,warnings,etc.AnimageoftheZBM2batteryisshowninthefollowingfigure.

TheL-shapedtankscontainingtheelectrolytearevisibleatthebasewhileyoucanseethe

electrodestackandtheMMSboxatthetop.

FlowBatteryCharacteristics

Initsgeneraldefinition,aflowbatteryfeaturesanelectrolyteflowingcontinuouslybetween

a storage compartment (i.e. one or more tanks) and a reaction chamber where the

electrodes are placed. Compared to a lead-acid battery, a flow battery contains more

componentsandgreaterdesigncomplexity,withmovingpartsandsophisticatedchemical

containment. These batteries are typically deployed in stationary energy storage

applicationsaschangestothesystemverticalalignmentorchangesinmomentumorinertia

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can negatively affect the electrolyte flow, and thus the battery’s electrical

performance.TheZBM2on-boardModuleManagementSystem(MMS),whichcontrolsand

safeguards the battery, adds an active element to each battery that is not present in

standard batteries. While it increases the battery’s complexity, the MMS provides

advantageous functions that enable the ZBM2 to be self-managing and to protect itself

against potential environmental or operational damage risks. Where a ZBM2 replaces a

conventionalbatteryinasystemdesign,itschargecycleisstartedbysimplypresentingan

appropriatechargingvoltageandcurrenttotheRedflowbattery.TheMMSrespondstothis

voltagebycommencingachargecycle.Likewise,adischargecycleiscommencedbysimply

drawing power from the battery, with the MMS managing the underlying active

componentstodelivertheenergytotheexternalloadasrequired.Acommoncharacteristic

of flowbatteries, including theZBM2, is that thebatterycancompletelydischargeduring

normal operation without incurring any damage or performance degradation. Unlike a

conventionalbattery,a completedischarge isharmless toaZBM2.However, this100per

centdischargecapability results in thebatteryoutputvoltage fallingtozerovoltsandthe

on-board MMS halting operation. This is a circumstance that a conventional battery

managementsystemmayreportasanalarmormayrespondbyhaltingsystemoperations.

Asnotedabove,afteracompletedischarge,theZBM2operationisre-commencedbysimply

applyinganappropriatechargevoltagetothesystem.Inresponse,theMMSre-startsand

commencesa ZBM2charging cycle.Consequently, thedesignof a system incorporatinga

ZBM2batteryorbatteriesneedstoaccommodatethis100percentdischargeandre-start

scenarioasanormalpartofbatteryoperations.

ZBM2BatteryAdvantages

Thevariousflowbatterycharacteristicsdiscussedinthissectioncanbeleveragedtodeliver

uniqueadvantagesinanenergystoragesystemusingtheZBM2zinc-bromineflowbattery.

Stackandtankseparation

Aswithmostflowbatteries,theRedflowZBM2featuresaseparationoftheenergysection-

the tanks filled with the electrolyte - and the power section - the stack composed by

electrodes.Thisseparationenablesuniqueadvantagesforflowbatteries,suchasflexibility

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indesignandrecyclingofthebatterypartsattheendoflife.Thus,theRedflow

ZBM2isoneofthemostsustainableandcost-effectivebatteriesavailabletoday.

Decreasedlikelihoodoftheft

AnotheradvantageoftheZBM2batteryoverlegacytechnologiessuchaslead-acidbatteries

is thereducedriskof theft in the field.Conventionalbatterybanks,whichcontainseveral

2V,6Vor12Vbatteries,areattractivetargetsfortheftasthesesingleelementscanbesold

orreused.TheuniquenessoftheZBM2designandcomponentsmeanthatthebatteryisof

little value to a potential thief outside of the application forwhich it has been designed.

Also,with a totalweight of about 240kg, the ZBM2 is too heavy for routine incidents of

opportunisticthefttowhichconventionalbatteriesareproneinremotefieldapplications.

Controlofimmediateshutdown

The separation of power and energy components in the ZBM2 battery also permits a

shutdown within seconds by interrupting power supply to the flow pumps. Since no

reactantsaresuppliedtotheelectrodesafterthepumpsstoprunning,noelectricalenergy

istransferred,sothesystembecomesstillandsafe.ThisenablesthesafetransportofZBM2

batteries in this shutdownstate.Also,aZBM2sufferingmechanicaldamagedoesnot risk

explosion,dangeroushighcurrentoutputfromshort-circuit,or“thermalrunaway”.

Inherentlysafecharacteristics

Thesecharacteristics,alongwithitswater-basedelectrolyte,makestheZBM2flowbattery

inherently safe, reducing to aminimum the risk of fire and “thermal runaway” in small,

mediumorlargeenergystoragesystems.Additionally,theelectrolyteflowdistributesheat

across the battery, contributing to easier thermalmanagement of the battery andmore

effectivecoolingcapabilities.

ModuleManagementSystem(MMS)

The ZBM2battery is equippedwith the RedflowBatteryManagement System (BMS) and

datacommunication,whichoffermultipleadvantages.

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Realtimedata,diagnosticsandeventhistory

Firstly, the ZBM2 can be accessed securely at any time to provide information about the

batterystatus,realtimedata,diagnosticandeventhistory.Thisaccessimprovescontrolof

the system,both localor remote,and itsperformance.These“smartbattery”capabilities

alsoimproveitsreliabilityandfaulttolerance.

MMSandBMSactivebatteryprotection

TheZBM2MMSprovidesactiveprotectionforthebatteryandthesystemagainstmisuse,

damage or extreme operating conditions, such as fluctuations in voltage or temperature.

Thisimprovessafetyandlongevityofboththesystemandthebatterybyminimisingfailures

andearlyreplacements.

Inaddition,theRedflowBMScaninterfacemultiplebatteriestoprovidefunctionslikedata

loggingand“clouding”,self-maintenancescheduling,eventresponse,chargeanddischarge

optimisation, fault diagnostics and signalling, auxiliary software-controlled I/Os

(Input/Output),invertercontrol,etc.ABMSinterfacescreenshotappearsbelow.

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100percentdepthofdischarge

The zinc-bromine flow battery’s 100 per cent depth of discharge delivers advantages in

manyenergy storage applicationswhereother batteries struggle.As thebattery can stay

fully or partially discharged for an indefinite timewithout degradation, it offers the best

choiceforapplicationsthatscheduletheuseofstoredenergyoroftenhaveidleperiods.

Zerostateofchargeadvantages

Insolarapplications,thebatterycanbestartedfromazeropercentstateofchargeevery

morningandusedtostoretheexcessenergysuppliedbythesunduringtheday.Theentire

batterycapacityisavailabletostoresolar-generatedenergy.Atanytimeduringthedayor

the night, the battery can be discharged partially or fully down to zero per cent state of

chargeandleftsafelyinthisconditionovernight.Ifnosolarenergyissuppliedtothebattery

thenextday,theflowbatteryincursno“ageing”orperformancedegradation.

Thiscapabilitycontrastswithothercommonbatterychemistriesthatmuststartthedayata

higherstateofcharge(forexample50percent)toavoiddeepdischargestatesthatcould

agethebatteryfaster.Thislimitstheiruseabledailyenergystoragecapacity.Whenstored

energy isneeded, thesebatteriescanbedischargedonlyback to50percent -no further

dischargeisallowed.Ifnosolarenergyisavailablethenextday,thebatterylifeandcapacity

isnegativelyaffectedbydeepdischargeconditionsleadingtosulfation.

The capability for safe and complete discharge of the ZBM2 battery also proves

advantageousindieselbackupgeneratorminimisationapplications.

Increasedcapacity

TheZBM2flowbattery’sabilitytouse100percentofitsstoredenergyalsoenablesbetter

sizingoftheenergystoragesystemcomparedtootherbatterytechnologies.Itisessentialto

factor this difference inuseable capacity for a ZBM2 into comparisons of battery system

capacity.A conventionalbatterymust reservea largeproportionof its theoretical energy

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storagecapacity toprotect thebattery from thedamageor reduceduseable

lifethatresultfrom100percentdepth-of-dischargeconditions.Aconventionalbatterycan

alsotakealongtimetochargefully,withcharge-ratesslowingasthebatteryapproachesits

completely full state. This means that even more theoretical battery capacity is often

reservedfromtheapplicationinpractice,toaccommodatethischaracteristic.Bycontrast,a

ZBM2canroutinelyoperatefromzeropercentto100percentstate-of-chargeandbackto

emptyforeveryoperationalcyclewithoutlossofperformanceorriskofbatterydamage.

Linearvoltage

Another advantage is that ZBM2 charge and discharge cycles operate at a near linear

voltageandcurrentlevelsacrosstheentirerangeofchargestates.ThisallowsaZBM2tobe

charged on a near-linear basis. It also allows the entire ZBM2 capacity to be usefully

delivered to the required system load, with output current and voltage also remaining

consistentandstableacrosstheentiredischargephase.ZBM2flowbatteriesarepreferable

in the case of remote applications where site access is difficult - for example, seasonal

impedimentsortransportdifficulties,etc.-becausetheycantoleratelongpoweroutages.

Completecelltestedperformanceandreliability

TheZBM2physicalandmechanicalcharacteristicsalsodeliverbenefits.Forexample,each

standard ZBM2 battery contains 30 cells in series directly interconnected to reach 48V

nominal voltage. Redflow tests every ZBM2 for the battery’s lifetime performance and

reliability.Othertechnologiestestonlythe individualcells - forexamplethe2Vcell inthe

lead-acidchemistry-butprovidenoinformationontheperformanceoftheinterconnected

24cellsthatareneededtoreachthefull48V.

Sealedassembly

The ZBM battery has a sealed assembly and uses corrosion-resistant materials. These

featuresmakethebatteryideallysuitedforextremeconditionenvironments.Asummaryof

RedflowbatteryadvantagesandchallengesisreportedinthefollowingTable1.

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ZBM2SelfMaintenanceCycles

Tooptimiseitsperformance,theZBM2mustbecompletelydischargedperiodicallyasaself-

maintenance requirement tomaximise thesurfaceareaof thebatteryelectrodesurfaces.

The ZBM2MMS automatically engages this ‘electrode scrubbing’ operation on a periodic

basisifthebatteryhasnotbeenrecentlydischargedtoazerostateofchargeduringnormal

operations. The ZBM2 system design needs to accommodate this typical flow battery

characteristic. For single battery systems, the designmust therefore accommodate short

periods where the ZBM2 is effectively ‘off’ at the end of a scrubbing cycle. For designs

containing multiple ZBM2s, batteries can be scheduled to undergo this cycling process

sequentially by accessing a menu in the BMS. The user can specify how to execute this

automatedself-maintenancesequencing–whetherallbatteriestogetheroroneatatime.

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TheZBM2Design

Thelargesizeoftypicalflowbatterydesignsmeansthatmostinstallationsworldwidefallin

the category of large-scale systems where the battery is custom-made for the specific

energystorageapplication.

Redflowhascommercialisedamodular,small-scalebatterythatenablesthedeploymentof

its ZBM2 flow battery for applications previously considered too small for flow batteries,

includingresidential,commercial&industrial(C&I)andtelecommunications.

AnexampleofthisscalabilityistheRedflowLSB(LargeScaleBattery)referenceplatform,a

shipping-container formfactorcontainingasmanyas60ZBM2batteries–or45batteries

andmultiplebatteryinverters–thatdeliverslarge-scaleenergystoragecapacitysuitablefor

renewables integration, distributed generation and uninterruptible power supply (UPS)

systems applications. In addition, the LSB itself is modular, allowing the deployment of

multipleLSBunitsforgrid-scaleenergystorageandload-levellingapplications.

Redflowmadekeydesignchoicestoensureonebatterycouldmatcheachofthosepotential

deploymentscenarios.Thefollowingsectiondiscussessomeofthesecrucialdesignchoices.

Mechanicaldesign

TheZBM2’sphysicalassemblyincludesaremovableelectrodestackplacedatthetopofthe

battery.Thisallowsforstackreplacementattheendoflifeandrecycling/refurbishmentof

thebattery.Twotanks–usingaone-inside-the-otherdesign-provide interstitialspaceto

simplify the battery assembly and contain potential electrolyte spillage. The sealedMMS

boxisatthefronttoavoiddirectexposureofbatteryterminalsandtoallowcablingaccess.

DCpumps

Redflow has co-developed a role-specific advanced DC pump for the ZBM2,which offers

several advantages. Firstly, the battery itself can power the pumps without requiring AC

components.Redflow’sDCpumpsfeaturebrushlessmotorsthatoffersuperiorperformance

in termsof life, noise, torque, reliability and robustness.An important feature is also the

capability todynamically adjust thepumpspeed, andhence the flow speed, according to

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therequiredchargeordischargerate.Thisdeliversanimprovementinoverall

batteryefficiencyandextendsthelifeofthepumps.TheRedflowMMSmonitorsthepump

flowrate,lettingyoudetectandreportpumpfailure.

Coolingapparatus

TheZBM2containsan internalcoolingapparatusthat isdesignedtoextendtheoperating

temperature range of the battery, allowing operations in the electrolyte temperature as

warmas50°Cwithoutanynegativeeffectonthebatterylife.

AdvancedMMS

As a plug-and-play energy storage device, Redflow’s ZBM2 is a “smart battery” that can

work independently from the surrounding system. Redflow achieves this by using an

advanced MMS that can perform independent operations and act when needed. The

RedflowBMScanalsostartsomeoperationswithexternalcommands,allowingthesystem

integratortoadjustordrivethesystemresponsesthroughsettingsintheBMSinterface.

Operationalfeatures

TheZBM2designisoptimisedfortelecommunicationapplicationswiththesefeatures:

• Voltage range adjusted for standard lead-acid systems with a maximum charging

voltageupto57Vandalowvoltagedisconnectdownto40V,

• Extendedhigh-endtemperaturerange,

• Noageingduetodeepdischargeandlongdischargedperiods,

• Lowmaintenance.

Redflow has included additional features for integrationwith renewables and interfacing

with advancedmonitoring. For example, Redflowhas introduced an electronic protection

for overcharge (not affecting the instantaneous discharge) for direct connection to solar

panels and renewables. Also, several ZBMs can be connected in parallelwithout adverse

inter-battery current flows due to differences in charge/discharge state. No protection

devicesarerequiredinthisconfiguration,andnounwantedflowofpowercanbeobserved

amongmultipleZBM2modulesconnectedtothesameDCbus.

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ZBM2 can be used with commercially available inverters for residential or

industrialapplicationstoprovidebackuporrenewablesshifting.ItiseasytointegrateZBM2

monitoringinmorecomplexsupervisorysystems.

ForlargeenergystorageapplicationsusingtheLSBreferenceplatform,ZBM2modulesare

connectedinseriestoreachhighervoltages.Thede-factostandardisthe400-800Vrange.

RedflowhasdesignedavariationoftheMMSmoduleforthisapplicationtoallowforseries

connectionwithoutaffectingtheZBM2protectionandaccesstobatteryinformation.

StandbyPowerSupplymode-ZBM2asapre-charged‘electro-chemical’generator

Oneof themost advantageousoperationalmodesof the ZBM2 is StandbyPower System

(SPS), a storage mode in which the battery can act as a pre-charged electro-chemical

generator.Requiringjust10percentofthebattery’sstoredenergytoestablish,thismode

enables you to leave the ZBM2unattended formonths,with no additional self-discharge

occurring.Thiseliminates theneed for tricklecharge,equalisationor conditioning.During

thisperiod,thevoltageonthebatteryterminalcandecaytozerovoltswhilethebatterywill

retainasmuchas90percentofitsoriginalstateofcharge.Whenrequired,thebatterycan

restartwithin 60 seconds - either by an external voltage of at least 36V or by restarting

itself.SPSmodeenablesthebatterytoself-sustainandsupplystoredenergy-asmuchas

9kWh from one ZBM2. If extra ZBM2s are connected in parallel, the first battery, once

restarted,canpoweruptheothers,withtheBMSrestartingtheminseconds.

TheZBM2inSPSmodecanbeusedinconjunctionwithotherbatteriesforUPSapplications.

Today, expensive lead-acid battery banks incur significant costs from ageing, power

consumption,weightandmaintenance.ZBM2alsoconsiderablyreducesoperationalcosts.

AnotherapplicationoftheZBM2’sSPSmodeisinareasthatexperiencefrequentblackouts.

Withoutrequiringanymaintenancecharge,theZBM2canstartintheeventofanelectricity

outage to power emergency loads. For example, with just one complete discharge each

week, the battery will only perform approximately 500 cycles during 10 years, providing

plentyofcapacitytosupplyStandbyPowerwhenrequired.TheZBM2batteryiswarranted

for10yearsor36,500cycles–whichevercomesfirst.

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LearnmoreaboutRedflowandZinc-BromineFlowBatteries

RedflowLimited,apublicly-listedAustraliancompany(ASX:RFX),producessmall10kWhzinc-

bromine flow batteries that tolerate daily hard work in harsh conditions. Marketed

asZCellandZBM2, Redflow batteries are designed for high cycle-rate, long time-base

stationary energy storage applications in the residential, commercial & industrial and

telecommunications sectors, and are scalable from a single battery installation through to

grid-scale deployments. Redflow batteries are sold, installed and maintained by an

internationalnetworkofenergysystemintegrators.Redflow’ssmart,self-protectingbatteries

offeruniqueadvantages includingsecureremotemanagement,100percentdailydepthof

discharge,toleranceofhighambienttemperatures,asimplerecyclingpath,nopropensityfor

thermalrunawayandsustainedenergydeliverythroughouttheiroperatinglife.

Redflowcontactdetails:

RedflowLimited(HeadOffice)

27CounihanRoad,

SeventeenMileRocks,Brisbane

QLD,4073,Australia

Phone:+61733760008

Fax:+61733763751

Sales:sales@redflow.com

General:info@redflow.com