Editor’s note: “Storing That Power” is a seven-part series detailing technologies capable of reserving power obtained from renewable sources. Read each week to learn more about pumped hydroelectric, industrial-scale batteries, flow batteries, flywheels, compressed air energy storage, gravel batteries and molten salt.

Many people are not familiar with flow batteries, which are a type of chemical battery well suited for large-scale power storage. Though the technology is fairly new, flow batteries currently offer a conversion efficiency between 65 and 75 percent, with the potential for increased efficiency as the technology develops.

Most batteries contain chemicals that react with one another to produce a charge, which is the source of electron flow. However, as reactions take place and the chemicals become depleted, it becomes harder and harder for a battery to provide the same level of energy. This wear signals a need for single-use batteries to be replaced and rechargeable batteries to be charged.

While some batteries can be drained fairly far and then recharged (known as deep-cycle batteries), depleting a battery too far tends to cause chemical reactions among the constituent materials that cannot be undone by recharging. Depletion can cause chemical crystallization, resulting in lessened battery capacity over time.

Batteries tend to lose some capacity as soon as they are put to use, because chemicals remain even after they react and become inactive. The inactive chemicals take up space in the battery and prevent some remaining active chemicals from interacting, thereby lessening capacity.

Now, imagine if you were able to connect a battery to a big tank full of reaction-producing chemicals, and that you could flush out depleted material and replace it with fresh material as the battery was being discharged. That, in essence, is how a flow battery operates. Rather than wrapping up all the material in cells, fresh material can flow into the battery cell as depleted chemicals flow out. When recharging the battery, the pumps are reversed. In this way, flow batteries are much like pumped hydroelectric systems; however, flow batteries function with chemicals in a compact space rather than using potential energy of water behind a dam.

Considering most of the flow battery chemicals sit in an inert tank, and are not in contact with any of the active parts of the battery, a flow battery can remain sitting without losing any of its charge. Likewise, flow batteries can be fully discharged and remain in that state for a long period of time without suffering damage when it is eventually recharged. Because the capacity of the battery is driven by the volume of its tanks, rather than needing to construct more complicated battery assemblies, it is relatively easy to add capacity to a system by increasing the size or number of tanks for the system.

At present, the most common form of flow batteries use vanadium in different oxide forms. Vanadium is a rare earth metal primarily used in industrial applications as an alloying element for strengthening steel. By using different oxides of vanadium with different charge states, the battery chemistry in vanadium flow batteries is simplified and does not suffer any long-term degradation or contamination of the electrolyte, since vanadium and vanadium oxides are the only chemicals in the system.

Non-vanadium flow batteries also exist, and some of those have the potential to provide increases in power density. As with other technologies that rely on scarce minerals, the global vanadium supply could play a role in further development of flow batteries, and whether or not they can be cost-effective in the future. The first mine in the United States to extract vanadium is under development in Nevada and is expected to begin production later this decade. The company behind this project is focusing on the battery market.

Flow batteries are a relatively new technology. Although originally dating back to the mid-1950s, most of the research leading to their development was carried out by NASA in the 1970s and by scientists at the University of New South Wales in the 1980s, where the contemporary, vanadium-based version was patented. The largest flow battery installation is a 1.5 megawatt facility at a semiconductor factory in Japan.

The technology offers relatively low energy density in terms of energy storage per pound of material–even compared to heavy storage systems like lead-acid batteries–which does not make them good candidates for mobile applications. For stationary power storage purposes, however, this is far less a critical factor than it is for vehicles or for portable electronics. Flow batteries’ high level of rechargeability and relative safety (compared to lead-acid batteries, which release dangerous hydrogen gas) and stability make them potentially well suited for further development for moderate-scale power storage systems.

Flow batteries are also useful as large-scale backup power supplies because of their ability to quickly respond (within fractions of a millisecond) to demand for power. Because of their long life expectancy, they would also do extremely well for load-shifting, where electricity is used to charge the battery during off-peak hours when rates are low, and then provide power back to a facility from the batteries instead of paying the premium, peak electricity rates.

Main image credit: University of New South Wales