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Improved Pv System Performance Using Vanadium Batteries

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  • 8/2/2019 Improved Pv System Performance Using Vanadium Batteries


    I M P R O V E D P V S Y S T E M P E R F O R M A N C E USING V A N A D I U M B A T T E R I E SRobert L. LargentDesign Assistance D ivisionCentre for Photovoltaic D evices and Systems

    University of New South Wales, Kensington 2033, AustraliaMaria Skylas-Kuacos artd John ChiengSchool of C hemistry and Industrial EngineeringUniversity of New South Wales, Kensington 2033, Australia

    ABSTRACTA Vanadium-Vanadium Redox battery can improvePhotovoltaic system performance, reliability and robustness byincreasing the energy conversion efficiency of the battery toX7%, by ma king the battery life, efficiency and ongoing energycapacity independent o f state of charge and load profiles and byreducing maintenance requirements. High battery efficiencyreduces the required PV while a battery life insensitive tobattery usage relaxes system constraints. These advantages areutilised in a demonstration PV system in Thailand that wasdesigned specifically to use vanadium technology, Following ii12 month field testing program with 4 kW Vanadium Batteries.300 systems consisting of 2-4 kW PV, a 4 kW , 15 kWhr.Vanadium Battery and a 4 kVA grid interactive inverter areintended to be installed in residences in Thailand.

    INTRODUCTIONIn PV applications requiring energy storage, the selection of theenergy storage system is of primary concern. The electricalparameters of the storage system constrain and shape the PVsystem . The deliverable power determines the maximum sizeof the electrical load, the energy storage capacity determines theduration of power to the load and energy conversion efficienc}determines the amount of extra PV needed to make up theenergy lost in the co nversion. Additionally, the reliability of thestorage system determincs if the PV system can be used in acritical application and maintenance scheduling determineswhen personnel must visit the PV site.PV systems engineers have traditionally employed electro-chemical storage using lead acid batteries. Extensivedevelopment and use of lead acid technology, particularly in theautomotive industry, has allowed the adaptation of that maturetechnology directly to PV applications. Lead acid technology iswell understood, is reliable, is in mass production and is readiljavailable; however. lead acid technology does have inherentattributes that must be designed around. In order to maintainenergy capacity and long battery life, extra energy must besupplied periodically to the battery to de-stratify the electrolyteand to equalise the cell voltages. This process of boos1charging causes hydrogen evolution and water loss from thebattery The additional energy associated with this process issupplied by the installation of extra PV and periodic

    maintenance is used replace the lost water. The battery life isstrongly affected by how discharged the battery is allowed toget before it is recharged and, if the battery is allowed to stay ina discharged state for very long, irreversible damage occurs tothr: plates of the battery. A useful battery parameter, the state ofcharge, is difficult to determine accurately and after the batteryis installed, it is, in practice, difficult to change the size of thebattery to account for the addition of new loads not specified forin the original system design.The constraints imposed by lead acid technology sugg est that amore flexible, higher efficiency and cost effective technologywould be a benefit to PV systems.A new type of electro-chemical storage developed by theUniversity of New South Wales (UNSW), the Vanadium-Vanadium Redox Battery [I] , exhibits many of the qualitiesdesired by PV systems designers. This battery has very higheficiency, a reasonable energy density, high charge anddischarge rates, a long lifespan independent of state of chargeand load profiles, and low maintenance requirements.These qualities greatly ease the constraints imposed upon PVsy:jtem engineers. It is not necessary to oversize the b attery inorder to maximise battery life or install additional PV for boostcharging. During periods of low sunlight, the battery can beoperated nominally at low states of charge with no effect uponbattery life.Additionally, this battery has a features which allows for manynew options not available with lead acid technology. It ispossible to simultaneously charge the battery at one voltagewhile discharging it at another voltage. This feature can beutilised to make a minimum cost, high efficiency, maximumpower point tracker or allows the battery to operate as a DCtransformer, electro-chemically transforming a current and avoltage into a different current and voltage.The Centre for Photovoltaic Devices and Systems incollaboration with the UNSW Vanadium Research Group andthe Thai Gypsum Products Co. Ltd., Thailand, has designedand installed a PV system using Vanadium Battery storage in ademonstration house in Thailand. This is a pre-commercialprototype version of a residential grid interactive systeminlended for installation in 30 0 houses in T hailand.

    11190-7803-1220-1193 $ 3 . 0 0 0 99 3 IEEE

  • 8/2/2019 Improved Pv System Performance Using Vanadium Batteries


    THE VANADIUM BATTERYRedox flow batteries employ a different energy conversionmethod than solid plate batteries. In contrast to the solid phasechemical changes that occ ur on the plates of a lead acid battery,a redox battery store s energy as chemical changes in two liquidelectrolytes that are hydraulically pumped through the batterystack. Energy conversion occurs in the battery stack and thecharged electrolyte is stored in reservoirs external to the batterystack. The physical size of the battery stack determines thepower available from the battery and the volume of theelectrolyte reservoirs determines the kWhrs energy storagt: ofthe battery.

    PUMP PUMPFig. 1. Schematic view of Vanadium Cell

    The development of the Vanadium-Vanadium Redox Battery atUNSW has ove rcome significant technical limitations that haveplagued other types of redox flow batteries.[2] The battery isinsensitive to atmospheric oxygen, has a high 1.4 V cellvoltage, has high longevity, low maintenance requirements andthe electrolytes are not mutually destructive.In the Vanadium battery, identical electrolyte is used initiall), inboth the positive and negative sides of the battery. Duringcharging, electro-chemical reactions within the battery stackchange the valance of the vanadium in the tw o electrolytes withthe negative reaction changing V(II1) to V(I1) and the positivereaction changin g V(IV) to V(V ). This process is reversedduring discharge. If any inadvertent mixing of the chargedelectrolytes occurs there is an energy loss as heat but, becausethe mixed ele ctrolytes revert back to their uncharged states, theycan be recharged next time through the stack. Thus, crosscontamination is not detrimental to the longevity of the battery.The above reactions do not, under all normal operatingconditions, generate hydrogen.Because both the valance reactions are permissible to theoriginal electrolyte, it is an arbitrary decision as to which sideof the battery is positive and which side of the battery isnegative. Only after initial charging is there a positive and anegative side of the battery.The battery stack electro-chemical reactions are all highlyefficient with the energy, voltage and coulombic efficiencies

    ranging from 90 to 99%. When the energy needed to operatethe pumps, which amounts to 2 to 3% of the total batteryenergy, is also take into account the total battery efficiency is avery high 87%.An accurate state of charge determination is made possible bymeasuring the open circuit voltage of a small vanadium cellattached to the battery with some portion of the electrolytesbeing pumped through it.These attributes of the Vanadium-Vanadium Redox Batterymake its use in PV systems very desirable.

    PV SYSTEMS WITH VANADIUM BATTERIESThe use of a Vanadium Battery with its very high energyconversion efficiency and no boost charge requirements directlyrelates to less PV being needed for the system. Greater systemrobustness is achieved through the battery's ability to be leftindefinitely at any state of charge with no reduction in batterylife and, because there is no hydrogen evolution, there is nowater loss from the electrolytes. Greater system flexibility isachieved with the new capability of tailoring the kWhrs storageto meet any new loads by varying the volume of the electrolytesand, because the electrolytes ar e stored separate from each other,there is very low self discharge. The battery itself can supplymultiple output voltages--a valuable advantage in PV systemswith DC loads of different voltage requirements. These featuresoffer new versatility in the choice of applications that use PVsystems. Material redundancy is minimised by the full power,very deep cycle (IOOYO)capability of the battery andadditionally, because there is no hydrogen evolution, there is noneed for forced ventilation. Maintenance requirements are lowwhich reduces visits to PV installations.Pump LossesThe 2-3% energy loss associatcd with the vanadium battery'spumps is calculated with the battery operating at full power. Ifthe battery is operating at low power then the pumping powerloss is a more significant proportion of the system power.The strategy to minimise this energy loss and improve systemefficiency is to turn the pumps off during periods of low chargeor discharge rates. With no electrolyte flow all of the powergoing into or coming from the battery operates directly on theelectrolytes present the stack.When the energy level of the stack electrolytes reaches athreshold, the pumps are turned on for a period of time whichfills the stack with fresh electrolytes, then the pumps are turnedoff and the battery again waits till the threshold is met.In PV systems where random load profiles are present thisfeature allows for an optimisation of pumping energy versessystem load power requirements. UNSW is developing amicro-controller based vanadium battery controller withstrategies for optimising the efficiency of the battery for theseapplications.


  • 8/2/2019 Improved Pv System Performance Using Vanadium Batteries


    The ability to easily and accurately determine the true state ofcharge of the Vanadium Battery allows for dynamic predictionsof the amount of time that a battery can sustain a load. Thisallows greater system diversity and gives the designer theability to fine tune the kW hr storage of the battery for differingload profiles and load types.Voltage TapsA valuable feature of the Vanadium Battery is its ability to h aveits charge voltage being different than its discharge voltage. Itis possible to simultaneously charge the battery at the 12 volttap and discharge at the 48 volt tap or visa versa. Used in thismanner, battery becomes an 87% efficient DC transformer.


    4 F - jFig. 2. Voltage taps for MPPT and differing chargeand discha rge voltages.

    Voltage taps increase system flexibility as loads with differentDC voltage requirements may be operated from one powersource without the additional conversion losses associated withvoltage matching.Maximum Power Point TrackingA maximum power point tracker (MPPT) is useful in reducingthe PV required in system applications. The relatively high costof the power electr onics MPPT, however, often reduces the costeffectiveness of the reduction of PV.The tap change method presents itself as being a highly costeffective and efficient method of Maximum Power PointTracking.PV array's maximum power point can be matched duringcharging by choosing an appropriate voltage tap on theVanadium Battery and changing to another voltage tap as PVarray's maximum power point changes. Unlike the complexpower electronics counterpart, there are no energy conversionlosses associated w ith the tap change method and the electronicsare relatively simple and rugged.

    ECONOMIC CONSIDERATIONS O F THE VANADIUMBATTERYAn economic analysis of Vanadium Storage technology hasdetermined the cost of Vanadium electrolytes to beUS$48/kWhr and the cost battery stack components to beUS$206/kW. Using a factor of 2.5 to account for the additionalcosts of storage tanks, etc., resulted in the capital cost for abattery varying from US$635/kWhr for a battery with 1 hourstorage capacity at full power discharge (e.g., 4 kW battery with4 kWhr s storage) to US$146/kWhr for a battery with 20 hoursof storage capacity at full power discharge (e.g., 4 kW batterywilh 80 kWh rs storage). This cost analysis indicates that thecost per kWhr is determined by the ratio of the battery's poweroulput to the number of total hours of full power storage. Thus,bolh a 1 kW battery with 20 kWhrs storage and a 4 MW batterywil h 80 MWhrs storage would be US$146/kWhr.

    Total Cost of Vanadium Storage perkWhr as a fuction of Storage Time

    600 !100 -I

    0 c .A + i +, -+ - ,-+-+,0 5 10 15 20

    STORAGE TIME IN HOURSFig. 3. Capital cost of Vanadium Battery per kWhr

    Most obvious in this analysis is the dramatic drop in cost perkPIhr as the battery goes from 1 to 5 hours of full powerstorage.A major economic advantage that Vanadium technology hasovI:r othe r technologie s relates to the ong oing costs of batterystorage. Because the electrolytes are not damaged byatmospheric oxygen or cross contamination they have anindefinite life and are considered to be a capital cost. Currentestimates indicate that the battery stack will need to be replacedevery five years yielding a relatively low ongoing cost, whencontrasted with a lead acid battery where, in PV applications,the entire battery needs to be replaced on the average of everyseven years.


  • 8/2/2019 Improved Pv System Performance Using Vanadium Batteries


    PV & VANADIUM DEMONSTRATION SYSTEM INTHAILANDThe first licensee for the commercialisation of the VanadiumBattery is the Thai Gypsum Products Co., Ltd., (TGP)Bangkok, Thailand . TGP built a PV & Vanadiumdemon stration house on their industrial estate at Laem Chabang,Thailand with the opening, lead by HRM Princess Maha ChacriSirindhorn of Thailand, on 23 December 1992. This functionhad 600 guests from industry, military and th e media.This PV & Vanadium system was installed in the demonstrationhouse in December 1992 by members of the UNSW Centre forPhotovoltaic Devices and Systems, UNSW Vanadium ResearchGroup, and the Thai Gypsum Vanadium CommercialisationGroup.The demonstration system was designed to operate AC loadsand a small, less than 800 watt, "compressor type" airconditione r was chosen as the load in the demonstration house.I

    1 kVA

    National Air-conCU-7OOK ditioner

    Fig. 4. Schematic of PV & Vanadium Battery systeminstalled in Thailand.Construction and assembly of the system was as follows:In Thailand, TGP built the demonstration house and roofmounted 36 Kyocera LA441K63 PV modules giving 2.2 kW ofinstalled PV .In Australia, the UNSW Centre for Photovoltaic Devices ilndSystems selected the inverter and other system components andbuilt the micro-controller for the Vanadium Battery as specifiedby the UNSW Vanadium Research Group.

    Butler Solar Products, Australia, the designers of the Siemens'range of SUNSME inverters, modified an existing 1 kW, 12volt stand alone SUNSINE inverter for the 16.8 volt PV &Vanadium system. This required a redesign of the transformer,installation of additional FET's in the bridge arms andmodifications for the Thai requirements of 220 V, 50Hz output.This inverter is not grid interactive.The system load is a National CU-700K split system"compressor type" air conditioner. The starting power requiredfor this air conditioner was measured to be from 6-11 kW--aconsiderable amount of peak power f or a 1 kW system.The initial charging of the Vanadium battery was with a powersupply connected to the AC grid.The demonstration system works as designed. Work continueswith this system giving the Thai Gypsum VanadiumCommercialisation Group hands on systems experience thatwill be directly applicable to their 300 house project.

    300 HOUSE PROJECTTGP is in the process of commercialising the 4 kW vanadiumbattery with the first application of the technology being a 300house installation in Bangkok. Each house will have a PV &Vanadium Battery system consisting of 2-4 kW PV, 4 kWVanadium Battery and a 4 quadrant, 4 kVA grid interactiveinverter. It is hoped that the first of the houses will be completeby the end of 1993 and that all 300 will be complete 18 monthslater. Data acquisition for system evaluation will be employed.

    CONCLUSIONThe Vanadium-Vanadium Redox Batteries offers systemperformance benefits though increased system efficiency androbustness, reduced maintenance requirements, and greaterflexibility in both system design and system application.The 300 house residential grid connected syste ms will test thisVanadium technology in a variety of system configurations.

    ACKNOWLEDGMENTSThe Centre for Photovoltaic Devices and Systems is supportedby the Australian Research Council under the Special ResearchCentre Scheme and by Pacific Power.Research for the Vanadium Battery development has beenfunded by ERDC, NS W Office of Energy and Thai GypsumProduct Co., Ltd. The support of Formica Australia insupplying material and fabricating endplates is also gratefullyacknowledge as is the assistance of Michael Kazacos, RuiHong, Dennis Yan and Jim Wilson.

    The UNSW Vanadium Research Group designed and built aVanadium battery rated at 1.2 kW, 15 kWhrs. This battery has12 cells, giving a system voltage of 16.8 Volt, and uses :!OOlitres in each of the two electrolyte reservoirs.


  • 8/2/2019 Improved Pv System Performance Using Vanadium Batteries


    REFERENCES[ 1 M. Skylas-Kazacos and R.G. R obins, "All Vanadium RedoxBattery", US Patent No . 4,786,567, 1986.

    [2] Maria Skylas-Kazacos, D. Kasherman, D.R. Hong, and M.Kamcos, "Characteristics an d Performance of a I kW UNSWVanadium Redox Battery", Journal of Power Sources, vol. 35.1991, pp 399-404.

    Fig. 5. 1.2 kW , 1 5 kWhr Vanadium Batteiy in Thai Demonstration


  • 8/2/2019 Improved Pv System Performance Using Vanadium Batteries


    Fig. 6. PV & Vanadium System in Demonstration House

    Fig. 7 . PV Modules on Demonstration House