IMPROVED PV SYSTEM PERFORMANCEUSING VANADIUM BATTERIES Robert L. Largent Design Assistance Division Centre for Photovoltaic Devices and Systems University of New South Wales, Kensington 2033, Australia Maria Skylas-Kuacos artd John Chieng School of Chemistry and Industrial Engineering University of New South Wales, Kensington 2033, Australia ABSTRACT A Vanadium-Vanadium Redox battery can improve Photovoltaic system performance, reliability and robustness by increasing the energy conversion efficiency of the battery to X7%, by making the battery life, efficiency and ongoing energy capacity independent of state of charge and load profiles and by reducing maintenance requirements. High battery efficiency reduces the required PV while a battery life insensitive to battery usage relaxes system constraints. These advantages are utilised in a demonstration PV system in Thailand that was designed specifically to use vanadium technology, Following ii 12 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 are intended to be installed in residences in Thailand. maintenance is used replace the lost water. The battery life is strongly affected by how discharged the battery is allowed to get before it is recharged and, if the battery is allowed to stay in a discharged state for very long, irreversible damage occurs to thr: plates of the battery. A useful battery parameter, the state of charge, is difficult to determine accurately and after the battery is installed, it is, in practice, difficult to change the size of the battery to account for the addition of new loads not specified for in the original system design. The constraints imposed by lead acid technology suggest that a more flexible, higher efficiency and cost effective technology would be a benefit to PV systems. A new type of electro-chemical storage developed by the University of New South Wales (UNSW), the VanadiumVanadium Redox Battery [I], exhibits many of the qualities desired by PV systems designers. This battery has very high eficiency, a reasonable energy density, high charge and discharge rates, a long lifespan independent of state of charge and load profiles, and low maintenance requirements. These qualities greatly ease the constraints imposed upon PV sy:jtem engineers. It is not necessary to oversize the battery in order to maximise battery life or install additional PV for boost charging. During periods of low sunlight, the battery can be operated nominally at low states of charge with no effect upon battery life. Additionally, this battery has a features which allows for many new options not available with lead acid technology. It is possible to simultaneously charge the battery at one voltage while discharging it at another voltage. This feature can be utilised to make a minimum cost, high efficiency, maximum power point tracker or allows the battery to operate as a DC transformer, electro-chemically transforming a current and a voltage into a different current and voltage. The Centre for Photovoltaic Devices and Systems in collaboration with the UNSW Vanadium Research Group and the Thai Gypsum Products Co. Ltd., Thailand, has designed and installed a PV system using Vanadium Battery storage in a demonstration house in Thailand. This is a pre-commercial prototype version of a residential grid interactive system inlended for installation in 300 houses in Thailand. INTRODUCTION In PV applications requiring energy storage, the selection of the energy storage system is of primary concern. The electrical parameters of the storage system constrain and shape the PV system. The deliverable power determines the maximum size of the electrical load, the energy storage capacity determines the duration of power to the load and energy conversion efficienc} determines the amount of extra PV needed to make up the energy lost in the conversion. Additionally, the reliability of the storage system determincs if the PV system can be used in a critical application and maintenance scheduling determines when personnel must visit the PV site. PV systems engineers have traditionally employed electrochemical storage using lead acid batteries. Extensive development and use of lead acid technology, particularly in the automotive industry, has allowed the adaptation of that mature technology directly to PV applications. Lead acid technology is well understood, is reliable, is in mass production and is readilj available; however. lead acid technology does have inherent attributes that must be designed around. In order to maintain energy capacity and long battery life, extra energy must be supplied periodically to the battery to de-stratify the electrolyte and to equalise the cell voltages. This process of “boos1 charging” causes hydrogen evolution and water loss from the battery The additional energy associated with this process is supplied by the installation of extra PV and periodic 1119 0-7803-1220-1193 $3.00 0 1993 IEEE electro-chemical reactions within the battery stack change the valance of the vanadium in the two electrolytes with the negative reaction changing V(II1) to V(I1) and the positive reaction changing V(IV) to V(V). the pumps are turned on for a period of time which fills the stack with fresh electrolytes. PUMP PUMP Fig. ranging from 90 to 99%.THE VANADIUM BATTERY Redox flow batteries employ a different energy conversion method than solid plate batteries. If the battery is operating at low power then the pumping power loss is a more significant proportion of the system power. Pump Losses The 2-3% energy loss associatcd with the vanadium battery's pumps is calculated with the battery operating at full power. UNSW is developing a micro-controller based vanadium battery controller with strategies for optimising the efficiency of the battery for these applications. When the energy level of the stack electrolytes reaches a threshold. a redox battery stores energy as chemical changes in two liquid electrolytes that are hydraulically pumped through the battery stack. there is no need for forced ventilation. under all normal operating conditions. Only after initial charging is there a positive and a negative side of the battery. which amounts to 2 to 3% of the total battery energy. because the mixed electrolytes revert back to their uncharged states. Material redundancy is minimised by the full power. it is an arbitrary decision as to which side of the battery is positive and which side of the battery is negative. If any inadvertent mixing of the charged electrolytes occurs there is an energy loss as heat but. in both the positive and negative sides of the battery. generate hydrogen. The battery itself can supply multiple output voltages--a valuable advantage in PV systems with DC loads of different voltage requirements. In PV systems where random load profiles are present this feature allows for an optimisation of pumping energy verses system load power requirements. With no electrolyte flow all of the power going into or coming from the battery operates directly on the electrolytes present the stack. because there is no hydrogen evolution. cross contamination is not detrimental to the longevity of the battery. These attributes of the Vanadium-Vanadium Redox Battery make its use in PV systems very desirable. When the energy needed to operate the pumps. identical electrolyte is used initiall). has high longevity. voltage and coulombic efficiencies 1120 . Energy conversion occurs in the battery stack and the charged electrolyte is stored in reservoirs external to the battery stack. The battery stack electro-chemical reactions are all highly efficient with the energy. very deep cycle (IOOYO) capability of the battery and additionally. Maintenance requirements are low which reduces visits to PV installations. because the electrolytes are stored separate from each other. Greater system robustness is achieved through the battery's ability to be left indefinitely at any state of charge with no reduction in battery life and. Thus. Schematic view of Vanadium Cell The development of the Vanadium-Vanadium Redox Battery at UNSW has overcome significant technical limitations that have plagued other types of redox flow batteries. In the Vanadium battery. Greater system flexibility is achieved with the new capability of tailoring the kWhrs storage to meet any new loads by varying the volume of the electrolytes and. has a high 1. During charging.4 V cell voltage. there is no water loss from the electrolytes. Because both the valance reactions are permissible to the original electrolyte. is also take into account the total battery efficiency is a very high 87%. In contrast to the solid phase chemical changes that occur on the plates of a lead acid battery. These features offer new versatility in the choice of applications that use PV systems. The strategy to minimise this energy loss and improve system efficiency is to turn the pumps off during periods of low charge or discharge rates. This process is reversed during discharge. low maintenance requirements and the electrolytes are not mutually destructive. PV SYSTEMS WITH VANADIUM BATTERIES The use of a Vanadium Battery with its very high energy conversion efficiency and no boost charge requirements directly relates to less PV being needed for the system. The above reactions do not.[2] The battery is insensitive to atmospheric oxygen. there is very low self discharge. they can be recharged next time through the stack. 1. An accurate state of charge determination is made possible by measuring the open circuit voltage of a small vanadium cell attached to the battery with some portion of the electrolytes being pumped through it. The physical size of the battery stack determines the power available from the battery and the volume of the electrolyte reservoirs determines the kWhrs energy storagt: of the battery. then the pumps are turned off and the battery again waits till the threshold is met. because there is no hydrogen evolution. . PV ARRAY ECONOMIC CONSIDERATIONS OF THE VANADIUM BATTERY An economic analysis of Vanadium Storage technology has determined the cost of Vanadium electrolytes to be US$48/kWhr and the cost battery stack components to be US$206/kW.. 3. Voltage taps for MPPT and differing charge and discharge voltages.. in PV applications. resulted in the capital cost for a battery varying from US$635/kWhr for a battery with 1 hour storage capacity at full power discharge (e.g. Used in this manner. Thus. Maximum Power Point Tracking A maximum power point tracker (MPPT) is useful in reducing 100 -I 0 c 0 /A . Voltage Taps A valuable feature of the Vanadium Battery is its ability to have its charge voltage being different than its discharge voltage.. This cost analysis indicates that the cost per kWhr is determined by the ratio of the battery's power oulput to the number of total hours of full power storage. 15 5 20 STORAGE TIME IN HOURS Fig. 2.-+10 . The tap change method presents itself as being a highly cost effective and efficient method of Maximum Power Point Tracking. Unlike the complex power electronics counterpart. however.++ . Capital cost of Vanadium Battery per kWhr Most obvious in this analysis is the dramatic drop in cost per kPIhr as the battery goes from 1 to 5 hours of full power storage.g. PV array's maximum power point can be matched during charging by choosing an appropriate voltage tap on the Vanadium Battery and changing to another voltage tap as PV array's maximum power point changes. bolh a 1 kW battery with 20 kWhrs storage and a 4 MW battery wil h 80 MWhrs storage would be US$146/kWhr.The ability to easily and accurately determine the true state of charge of the Vanadium Battery allows for dynamic predictions of the amount of time that a battery can sustain a load. etc. Voltage taps increase system flexibility as loads with different DC voltage requirements may be operated from one power source without the additional conversion losses associated with voltage matching. there are no energy conversion losses associated with the tap change method and the electronics are relatively simple and rugged. ovI:r other technologies relates to the ongoing costs of battery storage. 1121 . Using a factor of 2. Current estimates indicate that the battery stack will need to be replaced every five years yielding a relatively low ongoing cost. Total Cost of Vanadium Storage per kWhr as a fuction of Storage Time 600 ' 0 ° !7 4F-j Fig. This allows greater system diversity and gives the designer the ability to fine tune the kWhr storage of the battery for differing load profiles and load types. Because the electrolytes are not damaged by atmospheric oxygen or cross contamination they have an indefinite life and are considered to be a capital cost. when contrasted with a lead acid battery where. +-I i -+. the entire battery needs to be replaced on the average of every seven years. A major economic advantage that Vanadium technology has the PV required in system applications. 4 kW battery wilh 80 kWhrs storage). 4 kW battery with 4 kWhrs storage) to US$146/kWhr for a battery with 20 hours of storage capacity at full power discharge (e. The relatively high cost of the power electronics MPPT.5 to account for the additional costs of storage tanks.. battery becomes an 87% efficient DC transformer. often reduces the cost effectiveness of the reduction of PV. It is possible to simultaneously charge the battery at the 12 volt tap and discharge at the 48 volt tap or visa versa. This inverter is not grid interactive. 4 kVA grid interactive inverter.2 kW of installed PV..8 Volt. 300 HOUSE PROJECT I TGP is in the process of commercialising the 4 kW vanadium battery with the first application of the technology being a 300 house installation in Bangkok. 4. "compressor type" air conditioner was chosen as the load in the demonstration house. Thailand. Ltd. The initial charging of the Vanadium battery was with a power supply connected to the AC grid. lead by HRM Princess Maha Chacri Sirindhorn of Thailand. The 300 house residential grid connected systems will test this Vanadium technology in a variety of system configurations. military and the media. 12 volt stand alone SUNSINE inverter for the 16. reduced maintenance requirements. modified an existing 1 kW. Ltd. and uses :!OO litres in each of the two electrolyte reservoirs. Rui Hong.PV & VANADIUM DEMONSTRATION SYSTEM IN THAILAND The first licensee for the commercialisation of the Vanadium Battery is the Thai Gypsum Products Co. Butler Solar Products. 4 kW Vanadium Battery and a 4 quadrant. ACKNOWLEDGMENTS The Centre for Photovoltaic Devices and Systems is supported by the Australian Research Council under the Special Research Centre Scheme and by Pacific Power. The demonstration system was designed to operate AC loads and a small. TGP built the demonstration house and roof mounted 36 Kyocera LA441K63 PV modules giving 2. This required a redesign of the transformer.8 volt PV & Vanadium system.. This function had 600 guests from industry. installation of additional FET's in the bridge arms and modifications for the Thai requirements of 220 V. Research for the Vanadium Battery development has been funded by ERDC. giving a system voltage of 16. (TGP) Bangkok. Fig. TGP built a PV & Vanadium demonstration house on their industrial estate at Laem Chabang. Dennis Yan and Jim Wilson. Australia. and greater flexibility in both system design and system application. Work continues with this system giving the Thai Gypsum Vanadium Commercialisation Group hands on systems experience that will be directly applicable to their 300 house project. The support of Formica Australia in supplying material and fabricating endplates is also gratefully acknowledge as is the assistance of Michael Kazacos. This PV & Vanadium system was installed in the demonstration house in December 1992 by members of the UNSW Centre for Photovoltaic Devices and Systems. The demonstration system works as designed. NSW Office of Energy and Thai Gypsum Product Co. In Australia. on 23 December 1992. Thailand with the opening. the UNSW Centre for Photovoltaic Devices ilnd Systems selected the inverter and other system components and built the micro-controller for the Vanadium Battery as specified by the UNSW Vanadium Research Group. The system load is a National CU-700K split system "compressor type" air conditioner.2 kW. Each house will have a PV & Vanadium Battery system consisting of 2-4 kW PV. It is hoped that the first of the houses will be complete by the end of 1993 and that all 300 will be complete 18 months later. The starting power required for this air conditioner was measured to be from 6-11 kW--a considerable amount of peak power for a 1 kW system. the designers of the Siemens' range of SUNSME inverters. CONCLUSION 1 kVA National CU-7OOK Air-con ditioner The Vanadium-Vanadium Redox Batteries offers system performance benefits though increased system efficiency and robustness. I122 . 50 Hz output. 15 kWhrs. UNSW Vanadium Research Group. Schematic of PV & Vanadium Battery system installed in Thailand.. Construction and assembly of the system was as follows: In Thailand. The UNSW Vanadium Research Group designed and built a Vanadium battery rated at 1. and the Thai Gypsum Vanadium Commercialisation Group. less than 800 watt. This battery has 12 cells. Data acquisition for system evaluation will be employed. US Patent No.REFERENCES [ 11 M. 35.786. vol. 5. Kasherman.567. pp 399-404.2 kW.G. Fig. D.R. 4. 1986. Skylas-Kazacos and R. 15 kWhr Vanadium Batteiy in Thai Demonstration 1123 . f 1991. Hong. D. Kamcos. "All Vanadium Redox Battery". and M. Journal o Power Sources. 1. [2] Maria Skylas-Kazacos. Robins. "Characteristics and Performance of a I kW UNSW Vanadium Redox Battery". 7. PV Modules on Demonstration House 1124 . 6.Fig. PV & Vanadium System in Demonstration House Fig.
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