www.redflow.com
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
www.redflow.com
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
www.redflow.com
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
www.redflow.com
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.
www.redflow.com
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.
www.redflow.com
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
www.redflow.com
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.
www.redflow.com
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.
www.redflow.com
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
www.redflow.com
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.
www.redflow.com
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.
www.redflow.com
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:[email protected]
General:[email protected]