Flow Batteries

In a flow battery, two liquid electrolytes, acting as energy carriers, are stored in separate tanks (Figure 1). These liquid electrolytes are simultaneously pumped in a closed circuit through their respective half cells of the reaction cell, separated by an ion exchange membrane. The reaction cell contains the positive electrodes on the positive side and the negative electrode on the negative side of the flow battery. The ion exchange membrane prevents the electrolytes from mixing but allows selected ions to pass through to complete the reaction (Huskinson, 2013). “On charging, the electrical energy supplied causes a chemical reduction reaction in one electrolyte and an oxidation reaction in the other” (Woodbank Comm. Ltd, 2005). Flow batteries are often termed “redox” batteries because of this combined reduction reaction and oxidation reaction (Huskinson, 2013). On discharge this process is reversed and chemical energy is converted back into electrical energy.

Figure 1 showing a generic diagram of a flow battery.

There are many different chemistries for the electrolyte solutions that have proven viable, in some way, for flow batteries. Each chemistry places different requirements on the specific design of the flow battery. Each electrolyte chemistry also changes the values of the operating metrics of the energy storage device while driving the cost of the flow battery in a way unique to the chemicals used. Research and development in flow battery technology is focused on finding an economically viable electrolyte that produces a battery that operates under a profitable set of energy storage metrics (Huskinson, 2013). Some popular electrolyte chemistries that are current in production include vanadium, zinc bromine, polysulfide bromide and cerium zinc (Baxter, 2006).

Table 1 showing the description, metrics and considerations of common flow battery chemistries (Baxter, 2006) (Bradbury, 2010) (Huskinson, 2013).

Chemistry	Description	Metrics	Considerations
Vanadium	Each tank contains a different species of ionic vanadium in an aqueous sulfuric acid solution.	~85% efficiency Estimated 10,000 charge/discharge cycles (potential for recycling electrolyte) Estimated 7 to 15 year battery life System Cost estimate: $1828/kW and $783/kWh Electrolyte costs: $30-$50/kWh	Vanadium is currently a common alloying element in steel production. It must be mined and separated from other minerals. The cost of vanadium is one of the primary cost drivers for a vanadium redox battery.
Zinc Bromine	Each tank contains an aqueous solution of zinc and bromine that differ in their concentration of elemental bromine.	~80% efficiency Estimated 2,000 charge/discharge cycles Estimated 10 year battery life Cost estimate: $639/kW and $440–$485/kWh (for a 500 kW unit) Electrolyte costs: $10-$20/kWh	Electrodes can degrade rapidly over time, affecting performance.
Polysulfide Bromide	One tank contains sodium bromide while the other contains sodium polysulfide.	~60-65% efficiency Estimated 2,000 charge/discharge cycles Estimated 10-15 year battery life Cost estimate: $1400/kW and $200–$220/kWh Electrolyte costs: $10-$20/kWh	Bromine and hydrogen gas are emitted during the reaction. Sulfate crystals form on electrodes requiring regular maintenance.
Cerium Zinc	Negative tank contains zinc ions suspended in methane sulfonic acid (MSA). Positive tank contains cerium-MSA electrolyte.	~70% efficiency Estimated 15 year battery life Cost estimate: $750-$1000/kW Electrolyte costs: $50-$70/kWh	Requires separate tank for storing charged electrolyte and electrolyte awaiting charge.

Table 1 is in no way a comprehensive list of flow battery technologies. As with all forms of energy storage there is constant innovation and development. In November of 2012, a team from the Harvard School of Engineering and Applied Sciences (SEAS) was awarded a $600,000 innovation grant from the U.S. Department of Energy’s Advanced Research Projects Agency–Energy (ARPA-E) to further the development of a flow battery that uses a particular class of small organic molecules called quinones (Huskinson, 2013). Quinones are simple organic molecules that are part of the electron transport chain for photosynthesis.  According to a researcher in the project, Brian Huskinson, quinones are attractive electrolyte chemicals for several reasons. As an organic compound, quinones are abundant and can be synthesized in a process that is much easier and less environmentally impactful than mining and refining other chemicals such as vanadium. This could result in a comparatively low electrolyte price using a safer and more environmentally friendly chemical. Another advantage stems from the fact that the quinone based chemicals can be easily reformulated. Through experimentation, researchers seek to change the electrochemical potential of each electrolyte solution, hopefully resulting in greater electron flow (Huskinson, 2013). Though exact metrics are not yet available, the goal of the ARPA-E grant is to progress the research on this technology to determine if an economically viable flow battery can be developed for the purpose of renewable capacity firming (Huskinson, 2013).

One important characteristic of flow batteries is that the power rating and the energy rating are independent. The power rating of a flow battery scales with the size of the reaction cell (which contains the electrodes and ion exchange membrane). The energy rating of the flow battery scales with the amount of stored electrolyte. This provides flexibility in that the energy rating of an existing battery can be increased by adding additional tanks of electrolyte (Huskinson, 2013). Most flow batteries can be charged at the same time that they are being discharged, with some technologies having a charge time that is less than the discharge time (Baxter, 2006). The response time of flow batteries is commonly on the order or seconds driven mainly by the time it takes the pump to begin circulating the electrolyte (Huskinson, 2013). This disqualifies flow batteries for millisecond power quality applications. However, the potential to store large amounts of energy make flow batteries more appropriate for applications such as arbitrage and renewable capacity firming (EPRI, 2003). As with all energy storage technologies, media announcements on the topic of innovations in flow battery technology focus on their potential to mitigate the effect of the intermittency of renewables.

Works Cited

Baxter, R. (2006). Energy Storage; A Nontechnical Guide. Tulsa, Oklahoma: PennWell Corporation.

Bradbury, K. (2010). Energy Storage Technology Review. Durham, NC: Duke University.

EPRI. (2003). EPRI-DOE Handbook of Energy Storage for Transmission & Distribution Applications. Washington DC: EPRI, Palo Alto, CA, and the U.S. Department of Energy.

Huskinson, B. (2013, January 11). PhD candidate, Applied Physics; Harvard School of Engineering and Applied Sciences. (M. Banta, Interviewer)

Woodbank Comm. Ltd. (2005). Flow Batteries. Retrieved January 12, 2013, from Woodbank Communications Ltd.: http://www.mpoweruk.com/flow.htm