Redox Flow Battery
Flow Batteries like Redox Batteries are a specific subset of batteries. The fluid in the cells is stored and pumped during charge and discharge times for the chemical reaction to occur. Flow batteries are used mostly for long term discharge periods (greater than 5 hours) because the battery may be made larger simply by increasing the volume of the tanks storing the fluid. Although these batteries can be scaled larger relatively easily to increase the amount of energy in the system, there are some distinct disadvantages similar to pumped hydro rather than batteries. The increased system complexity requires higher amounts of construction and equipment. This causes more possible points of failure and thus increased maintenance costs in comparison to conventional batteries. In addition, the usage of the pump system results in more points for a reduction in the efficiency of the system.
Vanadium redox batteries (VRB) have been in development and are able for purchase and have been used in numerous scenarios. Of particular interest, VRB have been used for renewable integration (both solar and wind), load leveling, and power quality/reliability stabilization. The storage capacity of VRB is heavily dependent on the volume of the electrolyte within the battery. An example from the Purdue report cites a 10 MWh battery requiring between 12,000 and 17,000 square feet which is double the space requirement for a similarly sized conventional battery.
VRB generally have a large cycle life north of 10,000 cycles. This is approximated at 1,000 cycles per year from around 10 to 15 years. The equipment within the system may need more maintenance as discussed with the pumps and other equipment, but generally this life is quite long and not a problem. VRB efficiency however is generally lower at 60 to 70% in comparison to conventional batteries. This is the result of the additional equipment.
VRB are the more common flow battery, particularly due to simplicity and advantages of its construction. This is due to the chemicals being able to mix without contaminating each other. This would result in self-discharge of the VRB, but this is a smaller problem than other flow batteries. In addition, the chemicals used in the VRB is not poisonous or corrosive vapors. The membrane in the battery is toxic, but in general VRB are much easier for permits and siting as there are not emissions or fuel handling requirements associated with it.
One critical factor for competitiveness of this technology is its installed (capital) cost. ARPA-e targets capital costs of $100/kWh installed, an aggressive target for lowering the capital costs.
Load Leveling/ Peak Shaving Sample (Source:Climate Tech Wiki)
Both of the graphs pictured would be the load as seen by the grid/utility. The left graph is a sample load graph. On the right side, the impact of a large scale storage system (a Vanadium Redox Battery) is presented. The light pink portion is when the battery is being charged. The white tops of the load curve shows when the battery is discharging. If we discount the pink portion of the right graph, the result would be similar to peak shaving as the stored energy would need to have come from some other source.
Reaction Related Information
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Stoichiometry
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Temperature: Near room temperature (25⁰C)
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Concentration of vanadium: 1 M
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Concentration of H2SO4: 5 M
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Electrical power capacity : 1,000 kW
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Energy capacity: 12,000 kW-hr
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Cycle time (for charge or discharge): 12 hr
h. State of charge considerations: Minimum = 0.20, Maximum = 0.80 -
Average Potential of cell: 1.26 Volts (9)
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DC to DC efficiency: 0.91
B. Design Details
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Size (cross sectional area) of cell: 1 m2
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Cell stack size: 100 cells
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Design current density of cell: 40 mA/cm2
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Materials of construction for tanks and heat exchangers: PVC and high Ni steel
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Temperature adjustment in flow from cell stack: 15 ⁰C
C. Cost Information
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Industrial Grade Vanadium cost: $21.13/kg of V (10)
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Cell Construction Materials (11), Ion-exchange membrane: $500/m2, Electrodes: $51/m2, Carbon felt: $20/m2
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Costs are in 2011 U.S. dollars
The capital cost of this base case is about $4.5 million, or about $380 per kWh. The major cost associated with the energy storage capacity of the VRB is the cost of the vanadium electrolyte, which at the base case value represents about 40% of the capital costs of the entire battery (13). The large change in vanadium cost was used because it was apparent that the use of high purity vanadium at current prices (approximately $2,100 per kg) would be prohibitively expensive.
Aquino et al. (2017a) estimates that the fixed O&M for a vanadium redox flow battery system is somewhere between $7–$16/kW-yr and that the variable O&M cost is the same as other systems at $0.0003/kWh. Due to lack of information and reliability for O&M costs, the same O&M costs were used across all battery technologies as mentioned previously. The O&M costs are at least as high as other battery technologies due to the “growing pains” associated with a newly emerged technology.
External Revenue Streams: Aggregators
The National Renewable Energy Laboratory (NREL) collaborated with Sumitomo Electric to provide research support in modeling and optimally dispatching a utility-scale vanadium redox flow battery (VRFB) energy storage system. The primary objective of the project was to identify value streams through the application of utility-scale VRFB for local grid support use cases, including:
• Voltage regulation (droop): When operated in this mode, the system maintains the voltage on the feeder close to its nominal value. To accomplish this objective, the reactive power dispatched from the VRFB is based on the voltage at the battery’s point of common coupling using a voltage droop curve.
• Capacity firming: In this mode, the VRFB smooths high-frequency power flow fluctuations at the substation to a constant or low-frequency timescale average value.
• Peak shaving and valley filling: Peak shaving is defined as displacing the power consumption of the feeder by a predetermined amount for a specific time; this is a special case of load shifting. In this control mode, the VRFB is used to regulate the peak power of the feeder within a predefined limit.
• Energy arbitrage: To take advantage of the price difference of electricity across time periods, the VRFB is charged during off-peak hours and discharged during peak hours. The revenue obtained is the price differential between buying and selling electrical energy minus the cost of losses during the full charge/discharge cycle.
A further advantage is the simple recyclability of the batteries. Due to the high content of vanadium in the liquid electrolyte, the vanadium can easily be reintegrated into process chains and the existing value reused. The energy storage solution consists primarily of vanadium sulphate in a diluted (2mol/L) sulphuric acid containing a low concentration of phosphoric acid and is therefore roughly comparable to the acid of lead/acid batteries. The energy density is limited by the concentration of the pentavalent + VO2.