Ted Hesser Word Count = 796 excluding equations, title, figure captions and references
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Vanadium Redox Flow Battery Analysis for Renewable Energy Applications Introduction:
Intermitt Intermittent ent renewable renewable electricit electricity y generation generation (IREG) (IREG) presents presents significan significantt grid reliabili reliability ty issues issues for power systems engineers. Photovoltaic panels fluctuate power output as cloud cover and time of day varies spectral insolation flux. Wind turbines exhibit stochastic power output due to the mercurial nature of wind and the fact that the power output varies by the cube of the winds velocity. Increasing the use of IREG will require utilityscale scale batter battery y storag storagee infras infrastru tructu cture re to mainta maintain in grid grid reliab reliabili ility. ty. The Vanadi Vanadium um redox redox flow flow batter battery y (VRB) (VRB) complements IREG due to its overload capacity, its high storage efficiency, its ability to withstand a large number of deep charge/discharge cycles and its rapid response time. VRB Cell:
The VRB has two electrolyte loops both containing vanadium in sulfuric acid mediums, but in different valence states which may be oxidized/reduced at the electrodes. The vanadium redox pairs are V2+/V3+ and V4+/V5+ for negative and positive halves of the cell, respectively. The electrical balance is achieved by the transport of hydrogen ions in the electrolytes across the membrane during operation of the cell [1].
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The battery is fully rechargeable. If the electrolytes are are acci accide dent ntal allly mixe mixed d the the bat batter tery suff uffers ers no permanent damage [2]. The VRB’s energy is stored in the electrolyte, as opposed to the electrode material in conventional batteries. The capacity of a VRB cell is increased with the use of larger storage tanks. This allows the capacity of a VRB cell to be virtually limitless. If there is no power source, VRB’s can be recharged by replacing the electrolyte in the storage tanks [2]. Dramatic demand events can be managed by by refi refill llin ing g the the stor storage age tank tankss with with the the oxidi oxidize zed d vanadium species, thus instantaneously recharging the battery. battery. In the vanadium vanadium redox redox cell, the following following half-cell reactions are involved. At the negative electrode: V3+ + e- V2+ E0 = -0.26V
(1)
And at the positive electrode: Figure 1: Operating principle of the VRB cell [1]
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VO2+ + 2H+ + e- VO2+ + H20 E0 = 1.00V
(2)
The standard cell potential is thus E0 = 1.26 Volts. Under actual cell conditions, an open circuit voltage of 1.4 Volts is observed at 50% state of charge, while a fully charged cell produces over 1.6 Volts at opencircuit [2]. The VRBs state of charge (SOC) can be measured continuously via the Nerst equation [6]. QuickTimeª and a decompressor are needed to see this picture.
Figure 2: Components of a VRB cell [5]
(3)
Ted Hesser Word Count = 796 excluding equations, title, figure captions and references Consequently, the capacity remaining in the battery can be read instantly with battery stack voltage output. The power and voltage range of a VRB depends on the cell stack, while the energy capacity depends on the tank size [6]. The decoupling of voltage/power and capacity is useful for utility scale grid storage where specific voltage output is required for batteries of varying capacities.
Mat Sci 256
the cells do not measurably degrade in voltage output or efficiency over time.
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VRB vs. LAB
When compared with a traditional lead acid battery (LAB (LAB), ), a VRB VRB stan stands ds out out as supe superi rior or in ever every y categor category y except except for mass/v mass/volu olume me energy energy density density.. Theref Therefore ore VRBs VRBs are ideal ideal for utilit utility y scale scale storag storagee applications where large weight and volume are of little to no consequence.
Energy Density Wh/litre Power Density[W/kg] Temperature Temperature Range Efficiency Depth of Discharge Life cycle Maintenance Cost[$/kWh] Cost [$/kWh]
LAB 40
VRB 30
370 -5 to 400C 45% 25 to 30% 1500 $0.02
166 0 to 400C 80-90% 75% >10000 $0.008
$500-$1550
$300-$650
Figure Figure 2: Charge Charge Discharg Discharge e cyclic cyclic curves curves of VRB Stack [1]
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Figure 3: Comparison between LAB and VRB [4]
Charge/Discharge cycles
One problem associated with redox flow batteries is lifetime degradation caused by cross contamination of ions through through diffus diffusion ion across across the membra membrane. ne. This problem is solved in VRBs by using vanadyl sulphate solution in sulphuric acid as an electrolyte for both half half-c -cel ells ls [3]. [3]. By empl employ oyin ing g full fully y solu solubl blee redox redox couple coupless and inert inert electr electrodes odes,, undesir undesirabl ablee electr electrode ode pro proce cess sses es are are elim elimin inat ated ed and and thus thus ther theree are are no fund fundam amen enta tall cycl cyclee limi limita tati tion onss [3]. [3]. The The cycl cyclic ic performance of the VRB stack during deep charge/discharge cycles is very smooth indicating that
Figure 3: Coloumbic, voltage and energy efficiencies of 10kW class VRB stack [1]
System Response & Regulation Service:
The VRB system response time is less than 1ms and the the maxi maximu mum m shor shortt-ti time me overl overloa oad d outp output ut can can be several times that of rate capacity [4]. This makes the VRB an attractive option for providing voltage and frequency regulation services. Regulation services are contracted to fine-tune the voltage and frequency of the power system. Regulation services either produce or abso absorb rb powe powerr on the gri grid. The The U.S U.S. gri grid is synchronized to maintain a constant 60Hz frequency and too much generation or too little load causes the
Ted Hesser Word Count = 796 excluding equations, title, figure captions and references
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frequency to increase, and visa versa. Conclusion:
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VRB VRB tech techno nolo logy gy comp comple leme ment ntss IREG IREG due due to its its overlo overload ad capaci capacity, ty, its high high storag storagee effici efficiency ency,, its its ability to withst hstand a large num number ber of deep charge/discharge cycles and its rapid response time. VRBs VRBs are are supe superi rior or to LAB LABs for for grid grid stor torage age applications where the attributes of low mass/volume ener energy gy dens densit ity y are are of litt little le cons conseq equen uence ce.. VRBs VRBs exhi exhibi bitt exem exempl plar ary y cycl cyclic ic perf perfor orma manc ncee and and cell cell effi effici cienc encie iess by util utiliz izin ing g four four oxid oxidiz ized ed stat states es of Vanadium in the half-cell reactions, thus alleviating the the probl problem em of cros crosss cont contam amin inat atio ion n thro throug ugh h ion ion exch exchan ange ge acro across ss the the memb membra rane ne.. The The symb symbio iosi siss between VRBs and IREG lead me to the conclusion that both will be utilized as the world transitions from gree green-h n-hou ouse se-g -gas as emit emitti ting ng elec electr tric icit ity y gener generat atio ion n towards a cleaner and more sustainable future.
Figure 4: VRB overload capacity for different SOC [5]
VRBs are especially well suited for use with a wind generator because it can absorb fast fluctuations in wind power due to its fast response time and overload capacity [4]. Therefore, VRBs can be used to stabilize the intermittent nature of wind power generation.
References: [1] P. Zhao, H. Zhang, H. Zhou, J. Chen, S. Gao, B. Yi, “Characteristics “Characteristics and performance of 10kW class”, Journal of Power Sources 162 (2006) 1416–1420 [2] Ch. Fabjan, J. Garche, B. Harrer, L. Jo¨ rissen, C. Kolbeck, F. Philippi, G. Tomazic, F. Wagner, “The Vanadium RedoxBattery: an Efficient Storage Unit for Photovoltaic Systems,” Electrochimica Electrochimica Acta 47 (2001) 825–831.
[3] M Rychcik, M Skyllas-kazacos, “Characteristics of a New All-Vanadium All-Vanadium Redox Flow Battery”, Journal of Power Sources, 22 (1988) 59-67. QuickTimeª and a decompressor a r e n e e d e d t o s e e t h i s p ic ic t u r e .
[4] L. Barote, R. Weissbach, R. Teodorescu, C. Marinescu, Marinescu, M. Cirstea, “Stand-Alone Wind System with Vanadium Redox Battery Energy Storage”, IEEE. Power Engineering Society Summer Meeting 2001. [5] S. Miyake, N. Tokuda, Sumito Electric Industries, “Vanadium Redox-Flow Battery for a Variety of Applications”, Applications”, IEEE, Power Engineering Society Summer Meeting 2001.
Figur Figuree 5: Stabi Stabiliz lizati ation on of a wind wind turbin turbinee output output with 6 hr capacity VRB cell [5]
[6] L. Barote, C Marinescu, M. Georgescu, “VRB Modeling for storage in Stand-Alone Wind Energy Systems.” IEEE, Power Tech Conference Summer 2009
Ted Hesser Word Count = 796 excluding equations, title, figure captions and references
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