Advanced Battery Technologies for Stationary Energy Storage Applications: Page 4 of 5
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Generally speaking, flow batteries are categorized as either true redox flow batteries or hybrid redox flow batteries. The term redox is a contraction of reduction and oxidation (see “Energy Storage Glossary”), which describes the ion exchange at the core of the flow battery. In a true redox flow battery, the electrolyte chemicals remain dissolved and in solution at all times; in a hybrid redox flow battery, some of the chemicals that store electrochemical energy are plated as a solid.
One of the unique aspects of the true redox battery configuration is that it effectively decouples the battery’s energy rating from its power rating. The battery stores energy in the electrolyte itself, meaning that its energy rating increases or decreases in relation to tank volume. The power rating, meanwhile, is a function of cell stack area, meaning that it increases or decreases in relation to the number of cell stacks. By varying tank volume and the number of cell stacks, suppliers can tailor the energy and power ratings of a true redox flow battery to application-specific requirements.
While flow batteries do not provide the energy density of Li-ion devices, they have a much longer lifespan and present fewer safety concerns. Because the reactants are in the electrolyte, and the anode and cathode do not really participate in the chemical reaction, charge-discharge cycles do not age the electrodes, and battery capacity does not degrade. Because only a fraction of the electrolyte volume is in the battery cell stack at any one time, the short-circuit potential of a flow battery is negligible, typically posing no danger to equipment or personnel. Flow batteries also do not present a thermal runaway hazard, and the electrolyte is generally not flammable. Some flow batteries do not even require auxiliary cooling systems because the liquid electrolyte itself regulates temperatures inside the battery cell stack.
Though the hardware that makes up a flow battery is capital intensive, the product itself is relatively simple. Instead of having thousands of battery cells and cell-level management systems, the flow battery is more monolithic and has a lower parts count. Of course, moving parts such as pumps need replacing over the life of the system. In addition, even if the electrolyte is designed to last the service life of the battery, periodic electrolyte maintenance is required to reestablish the proper chemical balance and optimal fluid characteristics.
As is the case with Li-ion batteries, different flow battery chemistries have different profiles in terms of performance, toxicity, cost and so forth. Examples of flow battery chemistries deployed in grid applications include all-iron hybrid redox, vanadium redox, zinc-bromine hybrid redox and zinc-iron hybrid redox.
All-iron hybrid redox. In October 2012, Portland, Oregon–based Energy Storage Systems (ESS) received a nearly $3M ARPA-E award to commercialize a 10 kW/80 kWh all-iron hybrid redox flow battery. Part of the value proposition of an iron-based flow battery is that it uses abundant, low-cost materials that are environmentally benign. When charging, ferrous ions plate out as solid iron on the negative electrode; on discharge, the solid iron dissolves and releases two electrons to the positive electrode. The benefit of using the same element on both sides of the battery is that this eliminates degradation issues associated with cross-contamination of the electrolyte materials. An inherent challenge of an all-iron design is finding ways to improve power density. ESS has fielded its all-iron flow batteries in a variety of grid applications, including a 60 kW/225 kWh microgrid demonstration project at Fort Leonard Wood in Missouri.
Vanadium redox. Among the various flow battery chemistries, vanadium redox is the current market leader. The vanadium redox flow battery is a true redox flow device that uses vanadium-based electrolytes on both the positive and negative sides of the battery. During the discharge cycle, vanadium2+ ions oxidize in the negative electrode to form vanadium3+, allowing an electron to migrate to the positive electrode and reduce vanadium5+ ions to vanadium4+ ions, as shown in Figure 2 (p. 27). While vanadium is more expensive than iron, vanadium electrolytes provide a relatively higher cell voltage, which improves power and energy density. The downside of vanadium’s energetic nature is that the sulfuric acid–based electrolytes are corrosive, which exposes battery subcomponents to chemical stresses.
In the late 1980s, the University of New South Wales fielded the first vanadium redox battery. Companies have deployed the technology at scale globally in demonstration projects and in commercial applications for roughly a decade. Active vanadium redox flow battery companies include Sumitomo Electric, a large Japanese conglomerate; Vionx Energy, a Massachusetts-based start-up founded in 2015; and UniEnergy Technologies (UET), a Seattle-based company founded in 2012. In partnership with the US Department of Energy and a number of national laboratories, EPB, a municipally owned utility serving the greater Chattanooga, Tennessee, area, recently deployed a 100 kW/400 kWh vanadium redox flow battery from UET as part of a smart grid demonstration project. EPB will use the battery for renewables integration, voltage regulation, backup power and advanced microgrid operations and energy management.